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GREP ¶

grep prints lines that contain a match for one or more patterns.

This manual is for version 3.11 of GNU Grep.

This manual is for grep, a pattern matching engine.

> Permission is granted to copy, distribute and/or modify this document under
> the terms of the GNU Free Documentation License, Version 1.3 or any later
> version published by the Free Software Foundation; with no Invariant Sections,
> with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license
> is included in the section entitled “GNU Free Documentation License”.

TABLE OF CONTENTS

* 1 Introduction
* 2 Invoking grep
* 2.1 Command-line Options
* 2.1.1 Generic Program Information
* 2.1.2 Matching Control
* 2.1.3 General Output Control
* 2.1.4 Output Line Prefix Control
* 2.1.5 Context Line Control
* 2.1.6 File and Directory Selection
* 2.1.7 Other Options
* 2.2 Environment Variables
* 2.3 Exit Status
* 2.4 grep Programs
* 3 Regular Expressions
* 3.1 Fundamental Structure
* 3.2 Character Classes and Bracket Expressions
* 3.3 Special Backslash Expressions
* 3.4 Anchoring
* 3.5 Back-references and Subexpressions
* 3.6 Basic vs Extended Regular Expressions
* 3.7 Problematic Regular Expressions
* 3.8 Character Encoding
* 3.9 Matching Non-ASCII and Non-printable Characters
* 4 Usage
* 5 Performance
* 6 Reporting bugs
* 6.1 Known Bugs
* 7 Copying
* 7.1 GNU Free Documentation License
* Index

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1 INTRODUCTION ¶

Given one or more patterns, grep searches input files for matches to the
patterns. When it finds a match in a line, it copies the line to standard output
(by default), or produces whatever other sort of output you have requested with
options.

Though grep expects to do the matching on text, it has no limits on input line
length other than available memory, and it can match arbitrary characters within
a line. If the final byte of an input file is not a newline, grep silently
supplies one. Since newline is also a separator for the list of patterns, there
is no way to match newline characters in a text.

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2 INVOKING GREP ¶

The general synopsis of the grep command line is

grep [option...] [patterns] [file...]

There can be zero or more option arguments, and zero or more file arguments. The
patterns argument contains one or more patterns separated by newlines, and is
omitted when patterns are given via the ‘-e patterns’ or ‘-f file’ options.
Typically patterns should be quoted when grep is used in a shell command.

* Command-line Options
* Environment Variables
* Exit Status
* grep Programs

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2.1 COMMAND-LINE OPTIONS ¶

grep comes with a rich set of options: some from POSIX and some being GNU
extensions. Long option names are always a GNU extension, even for options that
are from POSIX specifications. Options that are specified by POSIX, under their
short names, are explicitly marked as such to facilitate POSIX-portable
programming. A few option names are provided for compatibility with older or
more exotic implementations.

Several additional options control which variant of the grep matching engine is
used. See grep Programs.

* Generic Program Information
* Matching Control
* General Output Control
* Output Line Prefix Control
* Context Line Control
* File and Directory Selection
* Other Options

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2.1.1 GENERIC PROGRAM INFORMATION ¶

--help

Print a usage message briefly summarizing the command-line options and the
bug-reporting address, then exit.

-V --version

Print the version number of grep to the standard output stream. This version
number should be included in all bug reports.

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2.1.2 MATCHING CONTROL ¶

-e patterns --regexp=patterns

Use patterns as one or more patterns; newlines within patterns separate each
pattern from the next. If this option is used multiple times or is combined with
the -f (--file) option, search for all patterns given. Typically patterns should
be quoted when grep is used in a shell command. (-e is specified by POSIX.)

-f file --file=file

Obtain patterns from file, one per line. If this option is used multiple times
or is combined with the -e (--regexp) option, search for all patterns given.
When file is ‘-’, read patterns from standard input. The empty file contains
zero patterns, and therefore matches nothing. (-f is specified by POSIX.)

-i -y --ignore-case

Ignore case distinctions in patterns and input data, so that characters that
differ only in case match each other. Although this is straightforward when
letters differ in case only via lowercase-uppercase pairs, the behavior is
unspecified in other situations. For example, uppercase “S” has an unusual
lowercase counterpart “ſ” (Unicode character U+017F, LATIN SMALL LETTER LONG S)
in many locales, and it is unspecified whether this unusual character matches
“S” or “s” even though uppercasing it yields “S”. Another example: the lowercase
German letter “ß” (U+00DF, LATIN SMALL LETTER SHARP S) is normally capitalized
as the two-character string “SS” but it does not match “SS”, and it might not
match the uppercase letter “ẞ” (U+1E9E, LATIN CAPITAL LETTER SHARP S) even
though lowercasing the latter yields the former.

-y is an obsolete synonym that is provided for compatibility. (-i is specified
by POSIX.)

--no-ignore-case

Do not ignore case distinctions in patterns and input data. This is the default.
This option is useful for passing to shell scripts that already use -i, in order
to cancel its effects because the two options override each other.

-v --invert-match

Invert the sense of matching, to select non-matching lines. (-v is specified by
POSIX.)

-w --word-regexp

Select only those lines containing matches that form whole words. The test is
that the matching substring must either be at the beginning of the line, or
preceded by a non-word constituent character. Similarly, it must be either at
the end of the line or followed by a non-word constituent character. Word
constituent characters are letters, digits, and the underscore. This option has
no effect if -x is also specified.

Because the -w option can match a substring that does not begin and end with
word constituents, it differs from surrounding a regular expression with ‘\<’
and ‘\>’. For example, although ‘grep -w @’ matches a line containing only ‘@’,
‘grep '\<@\>'’ cannot match any line because ‘@’ is not a word constituent. See
Special Backslash Expressions.

-x --line-regexp

Select only those matches that exactly match the whole line. For regular
expression patterns, this is like parenthesizing each pattern and then
surrounding it with ‘^’ and ‘$’. (-x is specified by POSIX.)

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2.1.3 GENERAL OUTPUT CONTROL ¶

-c --count

Suppress normal output; instead print a count of matching lines for each input
file. With the -v (--invert-match) option, count non-matching lines. (-c is
specified by POSIX.)

--color[=WHEN] --colour[=WHEN]

Surround matched non-empty strings, matching lines, context lines, file names,
line numbers, byte offsets, and separators (for fields and groups of context
lines) with escape sequences to display them in color on the terminal. The
colors are defined by the environment variable GREP_COLORS and default to
‘ms=01;31:mc=01;31:sl=:cx=:fn=35:ln=32:bn=32:se=36’ for bold red matched text,
magenta file names, green line numbers, green byte offsets, cyan separators, and
default terminal colors otherwise. See Environment Variables.

WHEN is ‘always’ to use colors, ‘never’ to not use colors, or ‘auto’ to use
colors if standard output is associated with a terminal device and the TERM
environment variable’s value suggests that the terminal supports colors. Plain
--color is treated like --color=auto; if no --color option is given, the default
is --color=never.

-L --files-without-match

Suppress normal output; instead print the name of each input file from which no
output would normally have been printed.

-l --files-with-matches

Suppress normal output; instead print the name of each input file from which
output would normally have been printed. Scanning each input file stops upon
first match. (-l is specified by POSIX.)

-m num --max-count=num

Stop after the first num selected lines. If num is zero, grep stops right away
without reading input. A num of −1 is treated as infinity and grep does not
stop; this is the default.

If the input is standard input from a regular file, and num selected lines are
output, grep ensures that the standard input is positioned just after the last
selected line before exiting, regardless of the presence of trailing context
lines. This enables a calling process to resume a search. For example, the
following shell script makes use of it:

while grep -m 1 'PATTERN'
do
echo xxxx
done < FILE

But the following probably will not work because a pipe is not a regular file:

# This probably will not work.
cat FILE |
while grep -m 1 'PATTERN'
do
echo xxxx
done

When grep stops after num selected lines, it outputs any trailing context lines.
When the -c or --count option is also used, grep does not output a count greater
than num. When the -v or --invert-match option is also used, grep stops after
outputting num non-matching lines.

-o --only-matching

Print only the matched non-empty parts of matching lines, with each such part on
a separate output line. Output lines use the same delimiters as input, and
delimiters are null bytes if -z (--null-data) is also used (see Other Options).

-q --quiet --silent

Quiet; do not write anything to standard output. Exit immediately with zero
status if any match is found, even if an error was detected. Also see the -s or
--no-messages option. Portability note: Solaris 10 grep lacks -q; portable shell
scripts typically can redirect standard output to /dev/null instead of using -q.
(-q is specified by POSIX.)

-s --no-messages

Suppress error messages about nonexistent or unreadable files. (-s is specified
by POSIX.)

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2.1.4 OUTPUT LINE PREFIX CONTROL ¶

When several prefix fields are to be output, the order is always file name, line
number, and byte offset, regardless of the order in which these options were
specified.

-b --byte-offset

Print the 0-based byte offset within the input file before each line of output.
If -o (--only-matching) is specified, print the offset of the matching part
itself.

-H --with-filename

Print the file name for each match. This is the default when there is more than
one file to search.

-h --no-filename

Suppress the prefixing of file names on output. This is the default when there
is only one file (or only standard input) to search.

--label=LABEL

Display input actually coming from standard input as input coming from file
LABEL. This can be useful for commands that transform a file’s contents before
searching; e.g.:

gzip -cd foo.gz | grep --label=foo -H 'some pattern'

-n --line-number

Prefix each line of output with the 1-based line number within its input file.
(-n is specified by POSIX.)

-T --initial-tab

Make sure that the first character of actual line content lies on a tab stop, so
that the alignment of tabs looks normal. This is useful with options that prefix
their output to the actual content: -H, -n, and -b. This may also prepend spaces
to output line numbers and byte offsets so that lines from a single file all
start at the same column.

-Z --null

Output a zero byte (the ASCII NUL character) instead of the character that
normally follows a file name. For example, ‘grep -lZ’ outputs a zero byte after
each file name instead of the usual newline. This option makes the output
unambiguous, even in the presence of file names containing unusual characters
like newlines. This option can be used with commands like ‘find -print0’, ‘perl
-0’, ‘sort -z’, and ‘xargs -0’ to process arbitrary file names, even those that
contain newline characters.

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2.1.5 CONTEXT LINE CONTROL ¶

Context lines are non-matching lines that are near a matching line. They are
output only if one of the following options are used. Regardless of how these
options are set, grep never outputs any given line more than once. If the -o
(--only-matching) option is specified, these options have no effect and a
warning is given upon their use.

-A num --after-context=num

Print num lines of trailing context after matching lines.

-B num --before-context=num

Print num lines of leading context before matching lines.

-C num -num --context=num

Print num lines of leading and trailing output context.

--group-separator=string

When -A, -B or -C are in use, print string instead of -- between groups of
lines.

--no-group-separator

When -A, -B or -C are in use, do not print a separator between groups of lines.

Here are some points about how grep chooses the separator to print between
prefix fields and line content:

* Matching lines normally use ‘:’ as a separator between prefix fields and
actual line content.
* Context (i.e., non-matching) lines use ‘-’ instead.
* When context is not specified, matching lines are simply output one right
after another.
* When context is specified, lines that are adjacent in the input form a group
and are output one right after another, while by default a separator appears
between non-adjacent groups.
* The default separator is a ‘--’ line; its presence and appearance can be
changed with the options above.
* Each group may contain several matching lines when they are close enough to
each other that two adjacent groups connect and can merge into a single
contiguous one.

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2.1.6 FILE AND DIRECTORY SELECTION ¶

-a --text

Process a binary file as if it were text; this is equivalent to the
‘--binary-files=text’ option.

--binary-files=type

If a file’s data or metadata indicate that the file contains binary data, assume
that the file is of type type. Non-text bytes indicate binary data; these are
either output bytes that are improperly encoded for the current locale (see
Environment Variables), or null input bytes when the -z (--null-data) option is
not given (see Other Options).

By default, type is ‘binary’, and grep suppresses output after null input binary
data is discovered, and suppresses output lines that contain improperly encoded
data. When some output is suppressed, grep follows any output with a message to
standard error saying that a binary file matches.

If type is ‘without-match’, when grep discovers null input binary data it
assumes that the rest of the file does not match; this is equivalent to the -I
option.

If type is ‘text’, grep processes binary data as if it were text; this is
equivalent to the -a option.

When type is ‘binary’, grep may treat non-text bytes as line terminators even
without the -z (--null-data) option. This means choosing ‘binary’ versus ‘text’
can affect whether a pattern matches a file. For example, when type is ‘binary’
the pattern ‘q$’ might match ‘q’ immediately followed by a null byte, even
though this is not matched when type is ‘text’. Conversely, when type is
‘binary’ the pattern ‘.’ (period) might not match a null byte.

Warning: The -a (--binary-files=text) option might output binary garbage, which
can have nasty side effects if the output is a terminal and if the terminal
driver interprets some of it as commands. On the other hand, when reading files
whose text encodings are unknown, it can be helpful to use -a or to set
‘LC_ALL='C'’ in the environment, in order to find more matches even if the
matches are unsafe for direct display.

-D action --devices=action

If an input file is a device, FIFO, or socket, use action to process it. If
action is ‘read’, all devices are read just as if they were ordinary files. If
action is ‘skip’, devices, FIFOs, and sockets are silently skipped. By default,
devices are read if they are on the command line or if the -R
(--dereference-recursive) option is used, and are skipped if they are
encountered recursively and the -r (--recursive) option is used. This option has
no effect on a file that is read via standard input.

-d action --directories=action

If an input file is a directory, use action to process it. By default, action is
‘read’, which means that directories are read just as if they were ordinary
files (some operating systems and file systems disallow this, and will cause
grep to print error messages for every directory or silently skip them). If
action is ‘skip’, directories are silently skipped. If action is ‘recurse’, grep
reads all files under each directory, recursively, following command-line
symbolic links and skipping other symlinks; this is equivalent to the -r option.

--exclude=glob

Skip any command-line file with a name suffix that matches the pattern glob,
using wildcard matching; a name suffix is either the whole name, or a trailing
part that starts with a non-slash character immediately after a slash (‘/’) in
the name. When searching recursively, skip any subfile whose base name matches
glob; the base name is the part after the last slash. A pattern can use ‘*’,
‘?’, and ‘[’...‘]’ as wildcards, and \ to quote a wildcard or backslash
character literally.

--exclude-from=file

Skip files whose name matches any of the patterns read from file (using wildcard
matching as described under --exclude).

--exclude-dir=glob

Skip any command-line directory with a name suffix that matches the pattern
glob. When searching recursively, skip any subdirectory whose base name matches
glob. Ignore any redundant trailing slashes in glob.

-I

Process a binary file as if it did not contain matching data; this is equivalent
to the ‘--binary-files=without-match’ option.

--include=glob

Search only files whose name matches glob, using wildcard matching as described
under --exclude. If contradictory --include and --exclude options are given, the
last matching one wins. If no --include or --exclude options match, a file is
included unless the first such option is --include.

-r --recursive

For each directory operand, read and process all files in that directory,
recursively. Follow symbolic links on the command line, but skip symlinks that
are encountered recursively. Note that if no file operand is given, grep
searches the working directory. This is the same as the ‘--directories=recurse’
option.

-R --dereference-recursive

For each directory operand, read and process all files in that directory,
recursively, following all symbolic links.

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2.1.7 OTHER OPTIONS ¶

Delimit the option list. Later arguments, if any, are treated as operands even
if they begin with ‘-’. For example, ‘grep PAT -- -file1 file2’ searches for the
pattern PAT in the files named -file1 and file2.

--line-buffered

Use line buffering for standard output, regardless of output device. By default,
standard output is line buffered for interactive devices, and is fully buffered
otherwise. With full buffering, the output buffer is flushed when full; with
line buffering, the buffer is also flushed after every output line. The buffer
size is system dependent.

-U --binary

On platforms that distinguish between text and binary I/O, use the latter when
reading and writing files other than the user’s terminal, so that all input
bytes are read and written as-is. This overrides the default behavior where grep
follows the operating system’s advice whether to use text or binary I/O. On
MS-Windows when grep uses text I/O it reads a carriage return–newline pair as a
newline and a Control-Z as end-of-file, and it writes a newline as a carriage
return–newline pair.

When using text I/O --byte-offset (-b) counts and --binary-files heuristics
apply to input data after text-I/O processing. Also, the --binary-files
heuristics need not agree with the --binary option; that is, they may treat the
data as text even if --binary is given, or vice versa. See File and Directory
Selection.

This option has no effect on GNU and other POSIX-compatible platforms, which do
not distinguish text from binary I/O.

-z --null-data

Treat input and output data as sequences of lines, each terminated by a zero
byte (the ASCII NUL character) instead of a newline. Like the -Z or --null
option, this option can be used with commands like ‘sort -z’ to process
arbitrary file names.

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2.2 ENVIRONMENT VARIABLES ¶

The behavior of grep is affected by several environment variables, the most
important of which control the locale, which specifies how grep interprets
characters in its patterns and data.

The locale for category LC_foo is specified by examining the three environment
variables LC_ALL, LC_foo, and LANG, in that order. The first of these variables
that is set specifies the locale. For example, if LC_ALL is not set, but
LC_COLLATE is set to ‘pt_BR.UTF-8’, then a Brazilian Portuguese locale is used
for the LC_COLLATE category. As a special case for LC_MESSAGES only, the
environment variable LANGUAGE can contain a colon-separated list of languages
that overrides the three environment variables that ordinarily specify the
LC_MESSAGES category. The ‘C’ locale is used if none of these environment
variables are set, if the locale catalog is not installed, or if grep was not
compiled with national language support (NLS). The shell command locale -a lists
locales that are currently available.

The following environment variables affect the behavior of grep.

GREP_COLOR

This obsolescent variable interacts with GREP_COLORS confusingly, and grep warns
if it is set and is not overridden by GREP_COLORS. Instead of
‘GREP_COLOR='color'’, you can use ‘GREP_COLORS='mt=color'’.

GREP_COLORS

This variable controls how the --color option highlights output. Its value is a
colon-separated list of terminfo capabilities that defaults to
‘ms=01;31:mc=01;31:sl=:cx=:fn=35:ln=32:bn=32:se=36’ with the ‘rv’ and ‘ne’
boolean capabilities omitted (i.e., false). The two-letter capability names
refer to terminal “capabilities,” the ability of a terminal to highlight text,
or change its color, and so on. These capabilities are stored in an online
database and accessed by the terminfo library. Non-empty capability values
control highlighting using Select Graphic Rendition (SGR) commands interpreted
by the terminal or terminal emulator. (See the section in the documentation of
your text terminal for permitted values and their meanings as character
attributes.) These substring values are integers in decimal representation and
can be concatenated with semicolons. grep takes care of assembling the result
into a complete SGR sequence (‘\33[’...‘m’). Common values to concatenate
include ‘1’ for bold, ‘4’ for underline, ‘5’ for blink, ‘7’ for inverse, ‘39’
for default foreground color, ‘30’ to ‘37’ for foreground colors, ‘90’ to ‘97’
for 16-color mode foreground colors, ‘38;5;0’ to ‘38;5;255’ for 88-color and
256-color modes foreground colors, ‘49’ for default background color, ‘40’ to
‘47’ for background colors, ‘100’ to ‘107’ for 16-color mode background colors,
and ‘48;5;0’ to ‘48;5;255’ for 88-color and 256-color modes background colors.

Supported capabilities are as follows.

sl=

SGR substring for whole selected lines (i.e., matching lines when the -v
command-line option is omitted, or non-matching lines when -v is specified). If
however the boolean ‘rv’ capability and the -v command-line option are both
specified, it applies to context matching lines instead. The default is empty
(i.e., the terminal’s default color pair).

cx=

SGR substring for whole context lines (i.e., non-matching lines when the -v
command-line option is omitted, or matching lines when -v is specified). If
however the boolean ‘rv’ capability and the -v command-line option are both
specified, it applies to selected non-matching lines instead. The default is
empty (i.e., the terminal’s default color pair).

Boolean value that reverses (swaps) the meanings of the ‘sl=’ and ‘cx=’
capabilities when the -v command-line option is specified. The default is false
(i.e., the capability is omitted).

mt=01;31

SGR substring for matching non-empty text in any matching line (i.e., a selected
line when the -v command-line option is omitted, or a context line when -v is
specified). Setting this is equivalent to setting both ‘ms=’ and ‘mc=’ at once
to the same value. The default is a bold red text foreground over the current
line background.

ms=01;31

SGR substring for matching non-empty text in a selected line. (This is used only
when the -v command-line option is omitted.) The effect of the ‘sl=’ (or ‘cx=’
if ‘rv’) capability remains active when this takes effect. The default is a bold
red text foreground over the current line background.

mc=01;31

SGR substring for matching non-empty text in a context line. (This is used only
when the -v command-line option is specified.) The effect of the ‘cx=’ (or ‘sl=’
if ‘rv’) capability remains active when this takes effect. The default is a bold
red text foreground over the current line background.

fn=35

SGR substring for file names prefixing any content line. The default is a
magenta text foreground over the terminal’s default background.

ln=32

SGR substring for line numbers prefixing any content line. The default is a
green text foreground over the terminal’s default background.

bn=32

SGR substring for byte offsets prefixing any content line. The default is a
green text foreground over the terminal’s default background.

se=36

SGR substring for separators that are inserted between selected line fields
(‘:’), between context line fields (‘-’), and between groups of adjacent lines
when nonzero context is specified (‘--’). The default is a cyan text foreground
over the terminal’s default background.

Boolean value that prevents clearing to the end of line using Erase in Line (EL)
to Right (‘\33[K’) each time a colorized item ends. This is needed on terminals
on which EL is not supported. It is otherwise useful on terminals for which the
back_color_erase (bce) boolean terminfo capability does not apply, when the
chosen highlight colors do not affect the background, or when EL is too slow or
causes too much flicker. The default is false (i.e., the capability is omitted).

Note that boolean capabilities have no ‘=’... part. They are omitted (i.e.,
false) by default and become true when specified.

LC_ALL LC_COLLATE LANG

These variables specify the locale for the LC_COLLATE category, which might
affect how range expressions like ‘a-z’ are interpreted.

LC_ALL LC_CTYPE LANG

These variables specify the locale for the LC_CTYPE category, which determines
the type of characters, e.g., which characters are whitespace. This category
also determines the character encoding. See Character Encoding.

LANGUAGE LC_ALL LC_MESSAGES LANG

These variables specify the locale for the LC_MESSAGES category, which
determines the language that grep uses for messages. The default ‘C’ locale uses
American English messages.

POSIXLY_CORRECT

If set, grep behaves as POSIX requires; otherwise, grep behaves more like other
GNU programs. POSIX requires that options that follow file names must be treated
as file names; by default, such options are permuted to the front of the operand
list and are treated as options.

TERM

This variable specifies the output terminal type, which can affect what the
--color option does. See General Output Control.

The GREP_OPTIONS environment variable of grep 2.20 and earlier is no longer
supported, as it caused problems when writing portable scripts. To make
arbitrary changes to how grep works, you can use an alias or script instead. For
example, if grep is in the directory ‘/usr/bin’ you can prepend $HOME/bin to
your PATH and create an executable script $HOME/bin/grep containing the
following:

#! /bin/sh
export PATH=/usr/bin
exec grep --color=auto --devices=skip "$@"

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2.3 EXIT STATUS ¶

Normally the exit status is 0 if a line is selected, 1 if no lines were
selected, and 2 if an error occurred. However, if the -q or --quiet or --silent
option is used and a line is selected, the exit status is 0 even if an error
occurred. Other grep implementations may exit with status greater than 2 on
error.

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2.4 GREP PROGRAMS ¶

grep searches the named input files for lines containing a match to the given
patterns. By default, grep prints the matching lines. A file named - stands for
standard input. If no input is specified, grep searches the working directory .
if given a command-line option specifying recursion; otherwise, grep searches
standard input. There are four major variants of grep, controlled by the
following options.

-G --basic-regexp

Interpret patterns as basic regular expressions (BREs). This is the default.

-E --extended-regexp

Interpret patterns as extended regular expressions (EREs). (-E is specified by
POSIX.)

-F --fixed-strings

Interpret patterns as fixed strings, not regular expressions. (-F is specified
by POSIX.)

-P --perl-regexp

Interpret patterns as Perl-compatible regular expressions (PCREs). PCRE support
is here to stay, but consider this option experimental when combined with the -z
(--null-data) option, and note that ‘grep -P’ may warn of unimplemented
features. See Other Options.

For documentation, refer to https://www.pcre.org/, with these caveats:

* ‘\d’ matches only the ten ASCII digits (and ‘\D’ matches the complement),
regardless of locale. Use ‘\p{Nd}’ to also match non-ASCII digits. (The
behavior of ‘\d’ and ‘\D’ is unspecified after in-regexp directives like
‘(?aD)’.)
* Although PCRE tracks the syntax and semantics of Perl’s regular expressions,
the match is not always exact. For example, Perl evolves and a Perl
installation may predate or postdate the PCRE2 installation on the same host,
or their Unicode versions may differ, or Perl and PCRE2 may disagree about an
obscure construct.
* By default, grep applies each regexp to a line at a time, so the ‘(?s)’
directive (making ‘.’ match line breaks) is generally ineffective. However,
with -z (--null-data) it can work:

$ printf 'a\nb\n' |grep -zP '(?s)a.b'
a
b

But beware: with the -z (--null-data) and a file containing no NUL byte, grep
must read the entire file into memory before processing any of it. Thus, it
will exhaust memory and fail for some large files.

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3 REGULAR EXPRESSIONS ¶

A regular expression is a pattern that describes a set of strings. Regular
expressions are constructed analogously to arithmetic expressions, by using
various operators to combine smaller expressions. grep understands three
different versions of regular expression syntax: basic (BRE), extended (ERE),
and Perl-compatible (PCRE). In GNU grep, basic and extended regular expressions
are merely different notations for the same pattern-matching functionality. In
other implementations, basic regular expressions are ordinarily less powerful
than extended, though occasionally it is the other way around. The following
description applies to extended regular expressions; differences for basic
regular expressions are summarized afterwards. Perl-compatible regular
expressions have different functionality, and are documented in the
pcre2syntax(3) and pcre2pattern(3) manual pages, but work only if PCRE is
available in the system.

* Fundamental Structure
* Character Classes and Bracket Expressions
* Special Backslash Expressions
* Anchoring
* Back-references and Subexpressions
* Basic vs Extended Regular Expressions
* Problematic Regular Expressions
* Character Encoding
* Matching Non-ASCII and Non-printable Characters

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3.1 FUNDAMENTAL STRUCTURE ¶

In regular expressions, the characters ‘.?*+{|()[\^$’ are special characters and
have uses described below. All other characters are ordinary characters, and
each ordinary character is a regular expression that matches itself.

The period ‘.’ matches any single character. It is unspecified whether ‘.’
matches an encoding error.

A regular expression may be followed by one of several repetition operators; the
operators beginning with ‘{’ are called interval expressions.

‘?’

The preceding item is optional and is matched at most once.

‘*’

The preceding item is matched zero or more times.

‘+’

The preceding item is matched one or more times.

‘{n}’

The preceding item is matched exactly n times.

‘{n,}’

The preceding item is matched n or more times.

‘{,m}’

The preceding item is matched at most m times. This is a GNU extension.

‘{n,m}’

The preceding item is matched at least n times, but not more than m times.

The empty regular expression matches the empty string. Two regular expressions
may be concatenated; the resulting regular expression matches any string formed
by concatenating two substrings that respectively match the concatenated
expressions.

Two regular expressions may be joined by the infix operator ‘|’. The resulting
regular expression matches any string matching either of the two expressions,
which are called alternatives.

Repetition takes precedence over concatenation, which in turn takes precedence
over alternation. A whole expression may be enclosed in parentheses to override
these precedence rules and form a subexpression. An unmatched ‘)’ matches just
itself.

Not every character string is a valid regular expression. See Problematic
Regular Expressions.

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3.2 CHARACTER CLASSES AND BRACKET EXPRESSIONS ¶

A bracket expression is a list of characters enclosed by ‘[’ and ‘]’. It matches
any single character in that list. If the first character of the list is the
caret ‘^’, then it matches any character not in the list, and it is unspecified
whether it matches an encoding error. For example, the regular expression
‘[0123456789]’ matches any single digit, whereas ‘[^()]’ matches any single
character that is not an opening or closing parenthesis, and might or might not
match an encoding error.

Within a bracket expression, a range expression consists of two characters
separated by a hyphen. It matches any single character that sorts between the
two characters, inclusive. In the default C locale, the sorting sequence is the
native character order; for example, ‘[a-d]’ is equivalent to ‘[abcd]’. In other
locales, the sorting sequence is not specified, and ‘[a-d]’ might be equivalent
to ‘[abcd]’ or to ‘[aBbCcDd]’, or it might fail to match any character, or the
set of characters that it matches might be erratic, or it might be invalid. To
obtain the traditional interpretation of bracket expressions, you can use the
‘C’ locale by setting the LC_ALL environment variable to the value ‘C’.

Finally, certain named classes of characters are predefined within bracket
expressions, as follows. Their interpretation depends on the LC_CTYPE locale;
for example, ‘[[:alnum:]]’ means the character class of numbers and letters in
the current locale.

‘[:alnum:]’

Alphanumeric characters: ‘[:alpha:]’ and ‘[:digit:]’; in the ‘C’ locale and
ASCII character encoding, this is the same as ‘[0-9A-Za-z]’.

‘[:alpha:]’

Alphabetic characters: ‘[:lower:]’ and ‘[:upper:]’; in the ‘C’ locale and ASCII
character encoding, this is the same as ‘[A-Za-z]’.

‘[:blank:]’

Blank characters: space and tab.

‘[:cntrl:]’

Control characters. In ASCII, these characters have octal codes 000 through 037,
and 177 (DEL). In other character sets, these are the equivalent characters, if
any.

‘[:digit:]’

Digits: 0 1 2 3 4 5 6 7 8 9.

‘[:graph:]’

Graphical characters: ‘[:alnum:]’ and ‘[:punct:]’.

‘[:lower:]’

Lower-case letters; in the ‘C’ locale and ASCII character encoding, this is a b
c d e f g h i j k l m n o p q r s t u v w x y z.

‘[:print:]’

Printable characters: ‘[:alnum:]’, ‘[:punct:]’, and space.

‘[:punct:]’

Punctuation characters; in the ‘C’ locale and ASCII character encoding, this is
! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~.

‘[:space:]’

Space characters: in the ‘C’ locale, this is tab, newline, vertical tab, form
feed, carriage return, and space. See Usage, for more discussion of matching
newlines.

‘[:upper:]’

Upper-case letters: in the ‘C’ locale and ASCII character encoding, this is A B
C D E F G H I J K L M N O P Q R S T U V W X Y Z.

‘[:xdigit:]’

Hexadecimal digits: 0 1 2 3 4 5 6 7 8 9 A B C D E F a b c d e f.

Note that the brackets in these class names are part of the symbolic names, and
must be included in addition to the brackets delimiting the bracket expression.

If you mistakenly omit the outer brackets, and search for say, ‘[:upper:]’, GNU
grep prints a diagnostic and exits with status 2, on the assumption that you did
not intend to search for the regular expression ‘[:epru]’.

Special characters lose their special meaning inside bracket expressions.

‘]’

ends the bracket expression if it’s not the first list item. So, if you want to
make the ‘]’ character a list item, you must put it first.

‘[.’

represents the open collating symbol.

‘.]’

represents the close collating symbol.

‘[=’

represents the open equivalence class.

‘=]’

represents the close equivalence class.

‘[:’

represents the open character class symbol, and should be followed by a valid
character class name.

‘:]’

represents the close character class symbol.

‘-’

represents the range if it’s not first or last in a list or the ending point of
a range. To make the ‘-’ a list item, it is best to put it last.

‘^’

represents the characters not in the list. If you want to make the ‘^’ character
a list item, place it anywhere but first.

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3.3 SPECIAL BACKSLASH EXPRESSIONS ¶

The ‘\’ character followed by a special character is a regular expression that
matches the special character. The ‘\’ character, when followed by certain
ordinary characters, takes a special meaning:

‘\b’

Match the empty string at the edge of a word.

‘\B’

Match the empty string provided it’s not at the edge of a word.

‘\<’

Match the empty string at the beginning of a word.

‘\>’

Match the empty string at the end of a word.

‘\w’

Match word constituent, it is a synonym for ‘[_[:alnum:]]’.

‘\W’

Match non-word constituent, it is a synonym for ‘[^_[:alnum:]]’.

‘\s’

Match whitespace, it is a synonym for ‘[[:space:]]’.

‘\S’

Match non-whitespace, it is a synonym for ‘[^[:space:]]’.

‘\]’

Match ‘]’.

‘\}’

Match ‘}’.

For example, ‘\brat\b’ matches the separate word ‘rat’, ‘\Brat\B’ matches
‘crate’ but not ‘furry rat’.

The behavior of grep is unspecified if a unescaped backslash is not followed by
a special character, a nonzero digit, or a character in the above list. Although
grep might issue a diagnostic and/or give the backslash an interpretation now,
its behavior may change if the syntax of regular expressions is extended in
future versions.

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3.4 ANCHORING ¶

The caret ‘^’ and the dollar sign ‘$’ are special characters that respectively
match the empty string at the beginning and end of a line. They are termed
anchors, since they force the match to be “anchored” to beginning or end of a
line, respectively.

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Expressions [Contents][Index]

3.5 BACK-REFERENCES AND SUBEXPRESSIONS ¶

The back-reference ‘\n’, where n is a single nonzero digit, matches the
substring previously matched by the nth parenthesized subexpression of the
regular expression. For example, ‘(a)\1’ matches ‘aa’. If the parenthesized
subexpression does not participate in the match, the back-reference makes the
whole match fail; for example, ‘(a)*\1’ fails to match ‘a’. If the parenthesized
subexpression matches more than one substring, the back-reference refers to the
last matched substring; for example, ‘^(ab*)*\1$’ matches ‘ababbabb’ but not
‘ababbab’. When multiple regular expressions are given with -e or from a file
(‘-f file’), back-references are local to each expression.

See Known Bugs, for some known problems with back-references.

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3.6 BASIC VS EXTENDED REGULAR EXPRESSIONS ¶

Basic regular expressions differ from extended regular expressions in the
following ways:

* The characters ‘?’, ‘+’, ‘{’, ‘|’, ‘(’, and ‘)’ lose their special meaning;
instead use the backslashed versions ‘\?’, ‘\+’, ‘\{’, ‘\|’, ‘$’, and ‘$’.
Also, a backslash is needed before an interval expression’s closing ‘}’.
* An unmatched ‘\)’ is invalid.
* If an unescaped ‘^’ appears neither first, nor directly after ‘$’ or ‘\|’,
it is treated like an ordinary character and is not an anchor.
* If an unescaped ‘$’ appears neither last, nor directly before ‘\|’ or ‘$’,
it is treated like an ordinary character and is not an anchor.
* If an unescaped ‘*’ appears first, or appears directly after ‘\(’ or ‘\|’ or
anchoring ‘^’, it is treated like an ordinary character and is not a
repetition operator.

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3.7 PROBLEMATIC REGULAR EXPRESSIONS ¶

Some strings are invalid regular expressions and cause grep to issue a
diagnostic and fail. For example, ‘xy\1’ is invalid because there is no
parenthesized subexpression for the back-reference ‘\1’ to refer to.

Also, some regular expressions have unspecified behavior and should be avoided
even if grep does not currently diagnose them. For example, ‘xy\0’ has
unspecified behavior because ‘0’ is not a special character and ‘\0’ is not a
special backslash expression (see Special Backslash Expressions). Unspecified
behavior can be particularly problematic because the set of matched strings
might be only partially specified, or not be specified at all, or the expression
might even be invalid.

The following regular expression constructs are invalid on all platforms
conforming to POSIX, so portable scripts can assume that grep rejects these
constructs:

* A basic regular expression containing a back-reference ‘\n’ preceded by fewer
than n closing parentheses. For example, ‘$a$\2’ is invalid.
* A bracket expression containing ‘[:’ that does not start a character class;
and similarly for ‘[=’ and ‘[.’. For example, ‘[a[:b]’ and ‘[a[:ouch:]b]’ are
invalid.

GNU grep treats the following constructs as invalid. However, other grep
implementations might allow them, so portable scripts should not rely on their
being invalid:

* Unescaped ‘\’ at the end of a regular expression.
* Unescaped ‘[’ that does not start a bracket expression.
* A ‘\{’ in a basic regular expression that does not start an interval
expression.
* A basic regular expression with unbalanced ‘$’ or ‘$’, or an extended
regular expression with unbalanced ‘(’.
* In the POSIX locale, a range expression like ‘z-a’ that represents zero
elements. A non-GNU grep might treat it as a valid range that never matches.
* An interval expression with a repetition count greater than 32767. (The
portable POSIX limit is 255, and even interval expressions with smaller
counts can be impractically slow on all known implementations.)
* A bracket expression that contains at least three elements, the first and
last of which are both ‘:’, or both ‘.’, or both ‘=’. For example, a non-GNU
grep might treat ‘[:alpha:]’ like ‘[[:alpha:]]’, or like ‘[:ahlp]’.

The following constructs have well-defined behavior in GNU grep. However, they
have unspecified behavior elsewhere, so portable scripts should avoid them:

* Special backslash expressions like ‘\b’, ‘\<’, and ‘\]’. See Special
Backslash Expressions.
* A basic regular expression that uses ‘\?’, ‘\+’, or ‘\|’.
* An extended regular expression that uses back-references.
* An empty regular expression, subexpression, or alternative. For example,
‘(a|bc|)’ is not portable; a portable equivalent is ‘(a|bc)?’.
* In a basic regular expression, an anchoring ‘^’ that appears directly after
‘$’, or an anchoring ‘$’ that appears directly before ‘$’.
* In a basic regular expression, a repetition operator that directly follows
another repetition operator.
* In an extended regular expression, unescaped ‘{’ that does not begin a valid
interval expression. GNU grep treats the ‘{’ as an ordinary character.
* A null character or an encoding error in either pattern or input data. See
Character Encoding.
* An input file that ends in a non-newline character, where GNU grep silently
supplies a newline.

The following constructs have unspecified behavior, in both GNU and other grep
implementations. Scripts should avoid them whenever possible.

* A backslash escaping an ordinary character, unless it is a back-reference
like ‘\1’ or a special backslash expression like ‘\<’ or ‘\b’. See Special
Backslash Expressions. For example, ‘\x’ has unspecified behavior now, and a
future version of grep might specify ‘\x’ to have a new behavior.
* A repetition operator that appears directly after an anchor, or at the start
of a complete regular expression, parenthesized subexpression, or
alternative. For example, ‘+|^*(+a|?-b)’ has unspecified behavior, whereas
‘\+|^\*(\+a|\?-b)’ is portable.
* A range expression outside the POSIX locale. For example, in some locales
‘[a-z]’ might match some characters that are not lowercase letters, or might
not match some lowercase letters, or might be invalid. With GNU grep it is
not documented whether these range expressions use native code points, or use
the collating sequence specified by the LC_COLLATE category, or have some
other interpretation. Outside the POSIX locale, it is portable to use
‘[[:lower:]]’ to match a lower-case letter, or ‘[abcdefghijklmnopqrstuvwxyz]’
to match an ASCII lower-case letter.

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3.8 CHARACTER ENCODING ¶

The LC_CTYPE locale specifies the encoding of characters in patterns and data,
that is, whether text is encoded in UTF-8, ASCII, or some other encoding. See
Environment Variables.

In the ‘C’ or ‘POSIX’ locale, every character is encoded as a single byte and
every byte is a valid character. In more-complex encodings such as UTF-8, a
sequence of multiple bytes may be needed to represent a character, and some
bytes may be encoding errors that do not contribute to the representation of any
character. POSIX does not specify the behavior of grep when patterns or input
data contain encoding errors or null characters, so portable scripts should
avoid such usage. As an extension to POSIX, GNU grep treats null characters like
any other character. However, unless the -a (--binary-files=text) option is
used, the presence of null characters in input or of encoding errors in output
causes GNU grep to treat the file as binary and suppress details about matches.
See File and Directory Selection.

Regardless of locale, the 103 characters in the POSIX Portable Character Set (a
subset of ASCII) are always encoded as a single byte, and the 128 ASCII
characters have their usual single-byte encodings on all but oddball platforms.

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3.9 MATCHING NON-ASCII AND NON-PRINTABLE CHARACTERS ¶

In a regular expression, non-ASCII and non-printable characters other than
newline are not special, and represent themselves. For example, in a locale
using UTF-8 the command ‘grep 'Λ ω'’ (where the white space between ‘Λ’ and the
‘ω’ is a tab character) searches for ‘Λ’ (Unicode character U+039B GREEK CAPITAL
LETTER LAMBDA), followed by a tab (U+0009 TAB), followed by ‘ω’ (U+03C9 GREEK
SMALL LETTER OMEGA).

Suppose you want to limit your pattern to only printable characters (or even
only printable ASCII characters) to keep your script readable or portable, but
you also want to match specific non-ASCII or non-null non-printable characters.
If you are using the -P (--perl-regexp) option, PCREs give you several ways to
do this. Otherwise, if you are using Bash, the GNU project’s shell, you can
represent these characters via ANSI-C quoting. For example, the Bash commands
‘grep $'Λ\tω'’ and ‘grep $'\u039B\t\u03C9'’ both search for the same
three-character string ‘Λ ω’ mentioned earlier. However, because Bash translates
ANSI-C quoting before grep sees the pattern, this technique should not be used
to match printable ASCII characters; for example, ‘grep $'\u005E'’ is equivalent
to ‘grep '^'’ and matches any line, not just lines containing the character ‘^’
(U+005E CIRCUMFLEX ACCENT).

Since PCREs and ANSI-C quoting are GNU extensions to POSIX, portable shell
scripts written in ASCII should use other methods to match specific non-ASCII
characters. For example, in a UTF-8 locale the command ‘grep "$(printf
'\316\233\t\317\211\n')"’ is a portable albeit hard-to-read alternative to
Bash’s ‘grep $'Λ\tω'’. However, none of these techniques will let you put a null
character directly into a command-line pattern; null characters can appear only
in a pattern specified via the -f (--file) option.

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4 USAGE ¶

Here is an example command that invokes GNU grep:

grep -i 'hello.*world' menu.h main.c

This lists all lines in the files menu.h and main.c that contain the string
‘hello’ followed by the string ‘world’; this is because ‘.*’ matches zero or
more characters within a line. See Regular Expressions. The -i option causes
grep to ignore case, causing it to match the line ‘Hello, world!’, which it
would not otherwise match.

Here is a more complex example, showing the location and contents of any line
containing ‘f’ and ending in ‘.c’, within all files in the current directory
whose names start with non-‘.’, contain ‘g’, and end in ‘.h’. The -n option
outputs line numbers, the -- argument treats any later arguments as file names
not options even if *g*.h expands to a file name that starts with ‘-’, and the
empty file /dev/null causes file names to be output even if only one file name
happens to be of the form ‘*g*.h’.

grep -n -- 'f.*\.c$' *g*.h /dev/null

Note that the regular expression syntax used in the pattern differs from the
globbing syntax that the shell uses to match file names.

See Invoking grep, for more details about how to invoke grep.

Here are some common questions and answers about grep usage.

1. How can I list just the names of matching files?

grep -l 'main' test-*.c

lists names of ‘test-*.c’ files in the current directory whose contents
mention ‘main’.

2. How do I search directories recursively?

grep -r 'hello' /home/gigi

searches for ‘hello’ in all files under the /home/gigi directory. For more
control over which files are searched, use find and grep. For example, the
following command searches only C files:

find /home/gigi -name '*.c' ! -type d \
-exec grep -H 'hello' '{}' +

This differs from the command:

grep -H 'hello' /home/gigi/*.c

which merely looks for ‘hello’ in non-hidden C files in /home/gigi whose
names end in ‘.c’. The find command line above is more similar to the
command:

grep -r --include='*.c' 'hello' /home/gigi

3. What if a pattern or file has a leading ‘-’? For example:

grep "$pattern" *

can behave unexpectedly if the value of ‘pattern’ begins with ‘-’, or if
the ‘*’ expands to a file name with leading ‘-’. To avoid the problem, you
can use -e for patterns and leading ‘./’ for files:

grep -e "$pattern" ./*

searches for all lines matching the pattern in all the working directory’s
files whose names do not begin with ‘.’. Without the -e, grep might treat
the pattern as an option if it begins with ‘-’. Without the ‘./’, there
might be similar problems with file names beginning with ‘-’.

Alternatively, you can use ‘--’ before the pattern and file names:

grep -- "$pattern" *

This also fixes the problem, except that if there is a file named ‘-’, grep
misinterprets the ‘-’ as standard input.

4. Suppose I want to search for a whole word, not a part of a word?

grep -w 'hello' test*.log

searches only for instances of ‘hello’ that are entire words; it does not
match ‘Othello’. For more control, use ‘\<’ and ‘\>’ to match the start and
end of words. For example:

grep 'hello\>' test*.log

searches only for words ending in ‘hello’, so it matches the word
‘Othello’.

5. How do I output context around the matching lines?

grep -C 2 'hello' test*.log

prints two lines of context around each matching line.

6. How do I force grep to print the name of the file?

Append /dev/null:

grep 'eli' /etc/passwd /dev/null

gets you:

/etc/passwd:eli:x:2098:1000:Eli Smith:/home/eli:/bin/bash

Alternatively, use -H, which is a GNU extension:

grep -H 'eli' /etc/passwd

7. Why do people use strange regular expressions on ps output?

ps -ef | grep '[c]ron'

If the pattern had been written without the square brackets, it would have
matched not only the ps output line for cron, but also the ps output line
for grep. Note that on some platforms, ps limits the output to the width of
the screen; grep does not have any limit on the length of a line except the
available memory.

8. Why does grep report “Binary file matches”?

If grep listed all matching “lines” from a binary file, it would probably
generate output that is not useful, and it might even muck up your display.
So GNU grep suppresses output from files that appear to be binary files. To
force GNU grep to output lines even from files that appear to be binary,
use the -a or ‘--binary-files=text’ option. To eliminate the “Binary file
matches” messages, use the -I or ‘--binary-files=without-match’ option.

9. Why doesn’t ‘grep -lv’ print non-matching file names?

‘grep -lv’ lists the names of all files containing one or more lines that
do not match. To list the names of all files that contain no matching
lines, use the -L or --files-without-match option.

10. I can do “OR” with ‘|’, but what about “AND”?

grep 'paul' /etc/motd | grep 'franc,ois'

finds all lines that contain both ‘paul’ and ‘franc,ois’.

11. Why does the empty pattern match every input line?

The grep command searches for lines that contain strings that match a
pattern. Every line contains the empty string, so an empty pattern causes
grep to find a match on each line. It is not the only such pattern: ‘^’,
‘$’, and many other patterns cause grep to match every line.

To match empty lines, use the pattern ‘^$’. To match blank lines, use the
pattern ‘^[[:blank:]]*$’. To match no lines at all, use an extended regular
expression like ‘a^’ or ‘$a’. To match every line, a portable script should
use a pattern like ‘^’ instead of the empty pattern, as POSIX does not
specify the behavior of the empty pattern.

12. How can I search in both standard input and in files?

Use the special file name ‘-’:

cat /etc/passwd | grep 'alain' - /etc/motd

13. Why can’t I combine the shell’s ‘set -e’ with grep?

The grep command follows the convention of programs like cmp and diff where
an exit status of 1 is not an error. The shell command ‘set -e’ causes the
shell to exit if any subcommand exits with nonzero status, and this will
cause the shell to exit merely because grep selected no lines, which is
ordinarily not what you want.

There is a related problem with Bash’s set -e -o pipefail. Since grep does
not always read all its input, a command outputting to a pipe read by grep
can fail when grep exits before reading all its input, and the command’s
failure can cause Bash to exit.

14. Why is this back-reference failing?

echo 'ba' | grep -E '(a)\1|b\1'

This outputs an error message, because the second ‘\1’ has nothing to refer
back to, meaning it will never match anything.

15. How can I match across lines?

Standard grep cannot do this, as it is fundamentally line-based. Therefore,
merely using the [:space:] character class does not match newlines in the
way you might expect.

With the GNU grep option -z (--null-data), each input and output “line” is
null-terminated; see Other Options. Thus, you can match newlines in the
input, but typically if there is a match the entire input is output, so
this usage is often combined with output-suppressing options like -q, e.g.:

printf 'foo\nbar\n' | grep -z -q 'foo[[:space:]]\+bar'

If this does not suffice, you can transform the input before giving it to
grep, or turn to awk, sed, perl, or many other utilities that are designed
to operate across lines.

16. What do grep, -E, and -F stand for?

The name grep comes from the way line editing was done on Unix. For
example, ed uses the following syntax to print a list of matching lines on
the screen:

global/regular expression/print
g/re/p

The -E option stands for Extended grep. The -F option stands for Fixed
grep;

17. What happened to egrep and fgrep?

7th Edition Unix had commands egrep and fgrep that were the counterparts of
the modern ‘grep -E’ and ‘grep -F’. Although breaking up grep into three
programs was perhaps useful on the small computers of the 1970s, egrep and
fgrep were deemed obsolescent by POSIX in 1992, removed from POSIX in 2001,
deprecated by GNU Grep 2.5.3 in 2007, and changed to issue obsolescence
warnings by GNU Grep 3.8 in 2022; eventually, they are planned to be
removed entirely.

If you prefer the old names, you can use your own substitutes, such as a
shell script named egrep with the following contents:

#!/bin/sh
exec grep -E "$@"

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5 PERFORMANCE ¶

Typically grep is an efficient way to search text. However, it can be quite slow
in some cases, and it can search large files where even minor performance
tweaking can help significantly. Although the algorithm used by grep is an
implementation detail that can change from release to release, understanding its
basic strengths and weaknesses can help you improve its performance.

The grep command operates partly via a set of automata that are designed for
efficiency, and partly via a slower matcher that takes over when the fast
matchers run into unusual features like back-references. When feasible, the
Boyer–Moore fast string searching algorithm is used to match a single fixed
pattern, and the Aho–Corasick algorithm is used to match multiple fixed
patterns.

Generally speaking grep operates more efficiently in single-byte locales, since
it can avoid the special processing needed for multi-byte characters. If your
patterns will work just as well that way, setting LC_ALL to a single-byte locale
can help performance considerably. Setting ‘LC_ALL='C'’ can be particularly
efficient, as grep is tuned for that locale.

Outside the ‘C’ locale, case-insensitive search, and search for bracket
expressions like ‘[a-z]’ and ‘[[=a=]b]’, can be surprisingly inefficient due to
difficulties in fast portable access to concepts like multi-character collating
elements.

Interval expressions may be implemented internally via repetition. For example,
‘^(a|bc){2,4}$’ might be implemented as ‘^(a|bc)(a|bc)((a|bc)(a|bc)?)?$’. A
large repetition count may exhaust memory or greatly slow matching. Even small
counts can cause problems if cascaded; for example, ‘grep -E
".*{10,}{10,}{10,}{10,}{10,}"’ is likely to overflow a stack. Fortunately,
regular expressions like these are typically artificial, and cascaded
repetitions do not conform to POSIX so cannot be used in portable programs
anyway.

A back-reference such as ‘\1’ can hurt performance significantly in some cases,
since back-references cannot in general be implemented via a finite state
automaton, and instead trigger a backtracking algorithm that can be quite
inefficient. For example, although the pattern ‘^(.*)\1{14}(.*)\2{13}$’ matches
only lines whose lengths can be written as a sum 15x + 14y for nonnegative
integers x and y, the pattern matcher does not perform linear Diophantine
analysis and instead backtracks through all possible matching strings, using an
algorithm that is exponential in the worst case.

On some operating systems that support files with holes—large regions of zeros
that are not physically present on secondary storage—grep can skip over the
holes efficiently without needing to read the zeros. This optimization is not
available if the -a (--binary-files=text) option is used (see File and Directory
Selection), unless the -z (--null-data) option is also used (see Other Options).

For efficiency grep does not always read all its input. For example, the shell
command ‘sed '/^...$/d' | grep -q X’ can cause grep to exit immediately after
reading a line containing ‘X’, without bothering to read the rest of its input
data. This in turn can cause sed to exit with a nonzero status because sed
cannot write to its output pipe after grep exits.

For more about the algorithms used by grep and about related string matching
algorithms, see:

* Aho AV. Algorithms for finding patterns in strings. In: van Leeuwen J.
Handbook of Theoretical Computer Science, vol. A. New York: Elsevier; 1990.
p. 255–300. This surveys classic string matching algorithms, some of which
are used by grep.
* Aho AV, Corasick MJ. Efficient string matching: an aid to bibliographic
search. CACM. 1975;18(6):333–40. https://doi.org/10.1145/360825.360855. This
introduces the Aho–Corasick algorithm.
* Boyer RS, Moore JS. A fast string searching algorithm. CACM.
1977;20(10):762–72. https://doi.org/10.1145/359842.359859. This introduces
the Boyer–Moore algorithm.
* Faro S, Lecroq T. The exact online string matching problem: a review of the
most recent results. ACM Comput Surv. 2013;45(2):13.
https://doi.org/10.1145/2431211.2431212. This surveys string matching
algorithms that might help improve the performance of grep in the future.
* Hakak SI, Kamsin A, Shivakumara P, Gilkar GA, Khan WZ, Imran M. Exact string
matching algorithms: survey issues, and future research directions. IEEE
Access. 2019;7:69614–37. https://doi.org/10.1109/ACCESS.2019.2914071. This
survey is more recent than Faro & Lecroq, and focuses on taxonomy instead of
performance.
* Hume A, Sunday D. Fast string search. Software Pract Exper.
1991;21(11):1221–48. https://doi.org/10.1002/spe.4380211105. This excellent
albeit now-dated survey aided the initial development of grep.

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Next: Copying, Previous: Performance, Up: grep [Contents][Index]

6 REPORTING BUGS ¶

Bug reports can be found at the GNU bug report logs for grep. If you find a bug
not listed there, please email it to bug-grep@gnu.org to create a new bug
report.

* Known Bugs

--------------------------------------------------------------------------------

Up: Reporting bugs [Contents][Index]

6.1 KNOWN BUGS ¶

Large repetition counts in the ‘{n,m}’ construct may cause grep to use lots of
memory. In addition, certain other obscure regular expressions require
exponential time and space, and may cause grep to run out of memory.

Back-references can greatly slow down matching, as they can generate
exponentially many matching possibilities that can consume both time and memory
to explore. Also, the POSIX specification for back-references is at times
unclear. Furthermore, many regular expression implementations have
back-reference bugs that can cause programs to return incorrect answers or even
crash, and fixing these bugs has often been low-priority: for example, as of
2021 the GNU C library bug database contained back-reference bugs 52, 10844,
11053, 24269 and 25322, with little sign of forthcoming fixes. Luckily,
back-references are rarely useful and it should be little trouble to avoid them
in practical applications.

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Next: Index, Previous: Reporting bugs, Up: grep [Contents][Index]

7 COPYING ¶

GNU grep is licensed under the GNU GPL, which makes it free software.

The “free” in “free software” refers to liberty, not price. As some GNU project
advocates like to point out, think of “free speech” rather than “free beer”. In
short, you have the right (freedom) to run and change grep and distribute it to
other people, and—if you want—charge money for doing either. The important
restriction is that you have to grant your recipients the same rights and impose
the same restrictions.

This general method of licensing software is sometimes called open source. The
GNU project prefers the term “free software” for reasons outlined at
https://www.gnu.org/philosophy/open-source-misses-the-point.html.

This manual is free documentation in the same sense. The documentation license
is included below. The license for the program is available with the source
code, or at https://www.gnu.org/licenses/gpl.html.

* GNU Free Documentation License

--------------------------------------------------------------------------------

Up: Copying [Contents][Index]

7.1 GNU FREE DOCUMENTATION LICENSE ¶

Version 1.3, 3 November 2008

Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.

0. PREAMBLE

The purpose of this License is to make a manual, textbook, or other
functional and useful document free in the sense of freedom: to assure
everyone the effective freedom to copy and redistribute it, with or without
modifying it, either commercially or noncommercially. Secondarily, this
License preserves for the author and publisher a way to get credit for
their work, while not being considered responsible for modifications made
by others.

This License is a kind of “copyleft”, which means that derivative works of
the document must themselves be free in the same sense. It complements the
GNU General Public License, which is a copyleft license designed for free
software.

We have designed this License in order to use it for manuals for free
software, because free software needs free documentation: a free program
should come with manuals providing the same freedoms that the software
does. But this License is not limited to software manuals; it can be used
for any textual work, regardless of subject matter or whether it is
published as a printed book. We recommend this License principally for
works whose purpose is instruction or reference.

1. APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work, in any medium, that
contains a notice placed by the copyright holder saying it can be
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A “Modified Version” of the Document means any work containing the Document
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A “Secondary Section” is a named appendix or a front-matter section of the
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The “Invariant Sections” are certain Secondary Sections whose titles are
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The Document may include Warranty Disclaimers next to the notice which
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2. VERBATIM COPYING

You may copy and distribute the Document in any medium, either commercially
or noncommercially, provided that this License, the copyright notices, and
the license notice saying this License applies to the Document are
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You may also lend copies, under the same conditions stated above, and you
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3. COPYING IN QUANTITY

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these conditions, can be treated as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit legibly,
you should put the first ones listed (as many as fit reasonably) on the
actual cover, and continue the rest onto adjacent pages.

If you publish or distribute Opaque copies of the Document numbering more
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with each Opaque copy, or state in or with each Opaque copy a
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latter option, you must take reasonably prudent steps, when you begin
distribution of Opaque copies in quantity, to ensure that this Transparent
copy will remain thus accessible at the stated location until at least one
year after the last time you distribute an Opaque copy (directly or through
your agents or retailers) of that edition to the public.

It is requested, but not required, that you contact the authors of the
Document well before redistributing any large number of copies, to give
them a chance to provide you with an updated version of the Document.

4. MODIFICATIONS

You may copy and distribute a Modified Version of the Document under the
conditions of sections 2 and 3 above, provided that you release the
Modified Version under precisely this License, with the Modified Version
filling the role of the Document, thus licensing distribution and
modification of the Modified Version to whoever possesses a copy of it. In
addition, you must do these things in the Modified Version:

A. Use in the Title Page (and on the covers, if any) a title distinct from
that of the Document, and from those of previous versions (which should,
if there were any, be listed in the History section of the Document).
You may use the same title as a previous version if the original
publisher of that version gives permission.
B. List on the Title Page, as authors, one or more persons or entities
responsible for authorship of the modifications in the Modified Version,
together with at least five of the principal authors of the Document
(all of its principal authors, if it has fewer than five), unless they
release you from this requirement.
C. State on the Title page the name of the publisher of the Modified
Version, as the publisher.
D. Preserve all the copyright notices of the Document.
E. Add an appropriate copyright notice for your modifications adjacent to
the other copyright notices.
F. Include, immediately after the copyright notices, a license notice
giving the public permission to use the Modified Version under the terms
of this License, in the form shown in the Addendum below.
G. Preserve in that license notice the full lists of Invariant Sections and
required Cover Texts given in the Document’s license notice.
H. Include an unaltered copy of this License.
I. Preserve the section Entitled “History”, Preserve its Title, and add to
it an item stating at least the title, year, new authors, and publisher
of the Modified Version as given on the Title Page. If there is no
section Entitled “History” in the Document, create one stating the
title, year, authors, and publisher of the Document as given on its
Title Page, then add an item describing the Modified Version as stated
in the previous sentence.
J. Preserve the network location, if any, given in the Document for public
access to a Transparent copy of the Document, and likewise the network
locations given in the Document for previous versions it was based on.
These may be placed in the “History” section. You may omit a network
location for a work that was published at least four years before the
Document itself, or if the original publisher of the version it refers
to gives permission.
K. For any section Entitled “Acknowledgements” or “Dedications”, Preserve
the Title of the section, and preserve in the section all the substance
and tone of each of the contributor acknowledgements and/or dedications
given therein.
L. Preserve all the Invariant Sections of the Document, unaltered in their
text and in their titles. Section numbers or the equivalent are not
considered part of the section titles.
M. Delete any section Entitled “Endorsements”. Such a section may not be
included in the Modified Version.
N. Do not retitle any existing section to be Entitled “Endorsements” or to
conflict in title with any Invariant Section.
O. Preserve any Warranty Disclaimers.

If the Modified Version includes new front-matter sections or appendices
that qualify as Secondary Sections and contain no material copied from the
Document, you may at your option designate some or all of these sections as
invariant. To do this, add their titles to the list of Invariant Sections
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You may add a section Entitled “Endorsements”, provided it contains nothing
but endorsements of your Modified Version by various parties—for example,
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You may add a passage of up to five words as a Front-Cover Text, and a
passage of up to 25 words as a Back-Cover Text, to the end of the list of
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and one of Back-Cover Text may be added by (or through arrangements made
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The author(s) and publisher(s) of the Document do not by this License give
permission to use their names for publicity for or to assert or imply
endorsement of any Modified Version.

5. COMBINING DOCUMENTS

You may combine the Document with other documents released under this
License, under the terms defined in section 4 above for modified versions,
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you preserve all their Warranty Disclaimers.

The combined work need only contain one copy of this License, and multiple
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titles in the list of Invariant Sections in the license notice of the
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In the combination, you must combine any sections Entitled “History” in the
various original documents, forming one section Entitled “History”;
likewise combine any sections Entitled “Acknowledgements”, and any sections
Entitled “Dedications”. You must delete all sections Entitled
“Endorsements.”

6. COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents
released under this License, and replace the individual copies of this
License in the various documents with a single copy that is included in the
collection, provided that you follow the rules of this License for verbatim
copying of each of the documents in all other respects.

You may extract a single document from such a collection, and distribute it
individually under this License, provided you insert a copy of this License
into the extracted document, and follow this License in all other respects
regarding verbatim copying of that document.

7. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate and
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distribution medium, is called an “aggregate” if the copyright resulting
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If the Cover Text requirement of section 3 is applicable to these copies of
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8. TRANSLATION

Translation is considered a kind of modification, so you may distribute
translations of the Document under the terms of section 4. Replacing
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Invariant Sections in addition to the original versions of these Invariant
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license notices in the Document, and any Warranty Disclaimers, provided
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original versions of those notices and disclaimers. In case of a
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If a section in the Document is Entitled “Acknowledgements”, “Dedications”,
or “History”, the requirement (section 4) to Preserve its Title (section 1)
will typically require changing the actual title.

9. TERMINATION

You may not copy, modify, sublicense, or distribute the Document except as
expressly provided under this License. Any attempt otherwise to copy,
modify, sublicense, or distribute it is void, and will automatically
terminate your rights under this License.

However, if you cease all violation of this License, then your license from
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until the copyright holder explicitly and finally terminates your license,
and (b) permanently, if the copyright holder fails to notify you of the
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Moreover, your license from a particular copyright holder is reinstated
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you cure the violation prior to 30 days after your receipt of the notice.

Termination of your rights under this section does not terminate the
licenses of parties who have received copies or rights from you under this
License. If your rights have been terminated and not permanently
reinstated, receipt of a copy of some or all of the same material does not
give you any rights to use it.

10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of the GNU
Free Documentation License from time to time. Such new versions will be
similar in spirit to the present version, but may differ in detail to
address new problems or concerns. See https://www.gnu.org/licenses/.

Each version of the License is given a distinguishing version number. If
the Document specifies that a particular numbered version of this License
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11. RELICENSING

“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World
Wide Web server that publishes copyrightable works and also provides
prominent facilities for anybody to edit those works. A public wiki that
anybody can edit is an example of such a server. A “Massive Multiauthor
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copyrightable works thus published on the MMC site.

“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license
published by Creative Commons Corporation, a not-for-profit corporation
with a principal place of business in San Francisco, California, as well as
future copyleft versions of that license published by that same
organization.

“Incorporate” means to publish or republish a Document, in whole or in
part, as part of another Document.

An MMC is “eligible for relicensing” if it is licensed under this License,
and if all works that were first published under this License somewhere
other than this MMC, and subsequently incorporated in whole or in part into
the MMC, (1) had no cover texts or invariant sections, and (2) were thus
incorporated prior to November 1, 2008.

The operator of an MMC Site may republish an MMC contained in the site
under CC-BY-SA on the same site at any time before August 1, 2009, provided
the MMC is eligible for relicensing.

ADDENDUM: HOW TO USE THIS LICENSE FOR YOUR DOCUMENTS ¶

To use this License in a document you have written, include a copy of the
License in the document and put the following copyright and license notices just
after the title page:

Copyright (C) year your name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts. A copy of the license is included in the section entitled ``GNU
Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace
the “with…Texts.” line with this:

with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.

If you have Invariant Sections without Cover Texts, or some other combination of
the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend
releasing these examples in parallel under your choice of free software license,
such as the GNU General Public License, to permit their use in free software.

--------------------------------------------------------------------------------

Previous: Copying, Up: grep [Contents][Index]

INDEX ¶

Jump to: - ? . { * +
A B C D E F G H I L M N O P Q R S T U V
W X Z

Index EntrySection

--------------------------------------------------------------------------------

- --Other Options --after-contextContext Line Control --basic-regexpgrep
Programs --before-contextContext Line Control --binaryOther Options
--binary-filesFile and Directory Selection --byte-offsetOutput Line Prefix
Control --colorGeneral Output Control --colourGeneral Output Control
--contextContext Line Control --countGeneral Output Control
--dereference-recursiveFile and Directory Selection --devicesFile and Directory
Selection --directoriesFile and Directory Selection --excludeFile and Directory
Selection --exclude-dirFile and Directory Selection --exclude-fromFile and
Directory Selection --extended-regexpgrep Programs --fileMatching Control
--files-with-matchesGeneral Output Control --files-without-matchGeneral Output
Control --fixed-stringsgrep Programs --group-separatorContext Line Control
--group-separatorContext Line Control --helpGeneric Program Information
--ignore-caseMatching Control --includeFile and Directory Selection
--initial-tabOutput Line Prefix Control --invert-matchMatching Control
--labelOutput Line Prefix Control --line-bufferedOther Options
--line-numberOutput Line Prefix Control --line-regexpMatching Control
--max-countGeneral Output Control --no-filenameOutput Line Prefix Control
--no-ignore-caseMatching Control --no-messagesGeneral Output Control
--nullOutput Line Prefix Control --null-dataOther Options --only-matchingGeneral
Output Control --perl-regexpgrep Programs --quietGeneral Output Control
--recursiveFile and Directory Selection --regexp=patternsMatching Control
--silentGeneral Output Control --textFile and Directory Selection
--versionGeneric Program Information --with-filenameOutput Line Prefix Control
--word-regexpMatching Control -AContext Line Control -aFile and Directory
Selection -bOutput Line Prefix Control -BContext Line Control -cGeneral Output
Control -CContext Line Control -DFile and Directory Selection -dFile and
Directory Selection -eMatching Control -Egrep Programs -fMatching Control -Fgrep
Programs -Ggrep Programs -HOutput Line Prefix Control -hOutput Line Prefix
Control -iMatching Control -LGeneral Output Control -lGeneral Output Control
-mGeneral Output Control -nOutput Line Prefix Control -numContext Line Control
-oGeneral Output Control -Pgrep Programs -qGeneral Output Control -rFile and
Directory Selection -RFile and Directory Selection -sGeneral Output Control
-TOutput Line Prefix Control -UOther Options -VGeneric Program Information
-vMatching Control -wMatching Control -xMatching Control -yMatching Control
-ZOutput Line Prefix Control -zOther Options

--------------------------------------------------------------------------------

? ?Fundamental Structure

--------------------------------------------------------------------------------

. .Fundamental Structure

--------------------------------------------------------------------------------

{ {,m}Fundamental Structure {n,}Fundamental Structure {n,m}Fundamental Structure
{n}Fundamental Structure

--------------------------------------------------------------------------------

* *Fundamental Structure

--------------------------------------------------------------------------------

+ +Fundamental Structure

--------------------------------------------------------------------------------

A after contextContext Line Control alnum character classCharacter Classes and
Bracket Expressions alpha character classCharacter Classes and Bracket
Expressions alphabetic charactersCharacter Classes and Bracket Expressions
alphanumeric charactersCharacter Classes and Bracket Expressions alternatives in
regular expressionsFundamental Structure anchoringAnchoring asteriskFundamental
Structure

--------------------------------------------------------------------------------

B back-referenceBack-references and Subexpressions back-referencesPerformance
backslashSpecial Backslash Expressions basic regular expressionsBasic vs
Extended before contextContext Line Control binary filesFile and Directory
Selection binary filesFile and Directory Selection binary I/OOther Options blank
character classCharacter Classes and Bracket Expressions blank
charactersCharacter Classes and Bracket Expressions bn GREP_COLORS
capabilityEnvironment Variables braces, first argument omittedFundamental
Structure braces, one argumentFundamental Structure braces, second argument
omittedFundamental Structure braces, two argumentsFundamental Structure bracket
expressionCharacter Classes and Bracket Expressions Bugs, knownKnown Bugs bugs,
reportingReporting Bugs byte offsetOutput Line Prefix Control

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C case insensitive searchMatching Control case insensitive searchPerformance
changing name of standard inputOutput Line Prefix Control character
classCharacter Classes and Bracket Expressions character classesCharacter
Classes and Bracket Expressions character encodingCharacter Encoding character
typeEnvironment Variables classes of charactersCharacter Classes and Bracket
Expressions cntrl character classCharacter Classes and Bracket Expressions
context linesGeneral Output Control context linesContext Line Control context
linesContext Line Control context lines, after matchContext Line Control context
lines, before matchContext Line Control control charactersCharacter Classes and
Bracket Expressions copyingCopying counting linesGeneral Output Control cx
GREP_COLORS capabilityEnvironment Variables

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D device searchFile and Directory Selection digit character classCharacter
Classes and Bracket Expressions digit charactersCharacter Classes and Bracket
Expressions directory searchFile and Directory Selection dotFundamental
Structure

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E encoding errorEnvironment Variables environment variablesEnvironment Variables
exclude directoriesFile and Directory Selection exclude filesFile and Directory
Selection exclude filesFile and Directory Selection exit statusExit Status

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F FAQ about grep usageUsage files which don’t matchGeneral Output Control fn
GREP_COLORS capabilityEnvironment Variables fn GREP_COLORS capabilityEnvironment
Variables

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G graph character classCharacter Classes and Bracket Expressions graphic
charactersCharacter Classes and Bracket Expressions grep programsgrep Programs
GREP_COLOR environment variableEnvironment Variables GREP_COLORS environment
variableEnvironment Variables group separatorContext Line Control group
separatorContext Line Control

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H hexadecimal digitsCharacter Classes and Bracket Expressions highlight
markersEnvironment Variables highlight markersEnvironment Variables highlight,
color, colourGeneral Output Control holes in filesPerformance

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I include filesFile and Directory Selection interval expressionsFundamental
Structure interval expressionsPerformance invalid regular expressionsProblematic
Expressions invert matchingMatching Control

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L LANG environment variableEnvironment Variables LANG environment
variableEnvironment Variables LANG environment variableEnvironment Variables
LANG environment variableEnvironment Variables LANGUAGE environment
variableEnvironment Variables LANGUAGE environment variableEnvironment Variables
language of messagesEnvironment Variables LC_ALL environment variableEnvironment
Variables LC_ALL environment variableEnvironment Variables LC_ALL environment
variableEnvironment Variables LC_ALL environment variableEnvironment Variables
LC_COLLATE environment variableEnvironment Variables LC_CTYPE environment
variableEnvironment Variables LC_MESSAGES environment variableEnvironment
Variables LC_MESSAGES environment variableEnvironment Variables line
bufferingOther Options line numberingOutput Line Prefix Control ln GREP_COLORS
capabilityEnvironment Variables localesPerformance lower character
classCharacter Classes and Bracket Expressions lower-case lettersCharacter
Classes and Bracket Expressions

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M match expression at most m timesFundamental Structure match expression at most
onceFundamental Structure match expression from n to m timesFundamental
Structure match expression n or more timesFundamental Structure match expression
n timesFundamental Structure match expression one or more timesFundamental
Structure match expression zero or more timesFundamental Structure match the
whole lineMatching Control matching basic regular expressionsgrep Programs
matching extended regular expressionsgrep Programs matching fixed stringsgrep
Programs matching Perl-compatible regular expressionsgrep Programs matching
whole wordsMatching Control max-countGeneral Output Control mc GREP_COLORS
capabilityEnvironment Variables message languageEnvironment Variables ms
GREP_COLORS capabilityEnvironment Variables MS-Windows binary I/OOther Options
mt GREP_COLORS capabilityEnvironment Variables

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N names of matching filesGeneral Output Control national language
supportEnvironment Variables national language supportEnvironment Variables ne
GREP_COLORS capabilityEnvironment Variables NLSEnvironment Variables no filename
prefixOutput Line Prefix Control non-ASCII matchingMatching Non-ASCII
non-printable matchingMatching Non-ASCII null characterEnvironment Variables
numeric charactersCharacter Classes and Bracket Expressions

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O only matchingGeneral Output Control option delimiterOther Options ordinary
charactersFundamental Structure

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P patterns from fileMatching Control patterns optionMatching Control
performancePerformance periodFundamental Structure pipelines and
readingPerformance plus signFundamental Structure POSIXLY_CORRECT environment
variableEnvironment Variables print character classCharacter Classes and Bracket
Expressions print non-matching linesMatching Control printable
charactersCharacter Classes and Bracket Expressions punct character
classCharacter Classes and Bracket Expressions punctuation charactersCharacter
Classes and Bracket Expressions

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Q question markFundamental Structure quiet, silentGeneral Output Control

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R range expressionCharacter Classes and Bracket Expressions recursive searchFile
and Directory Selection recursive searchFile and Directory Selection regular
expressionsRegular Expressions return statusExit Status rv GREP_COLORS
capabilityEnvironment Variables

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S searching directory treesFile and Directory Selection searching directory
treesFile and Directory Selection searching directory treesFile and Directory
Selection searching directory treesFile and Directory Selection searching
directory treesFile and Directory Selection searching for patternsIntroduction
sl GREP_COLORS capabilityEnvironment Variables space character classCharacter
Classes and Bracket Expressions space charactersCharacter Classes and Bracket
Expressions special charactersFundamental Structure subexpressionBack-references
and Subexpressions suppress binary dataFile and Directory Selection suppress
error messagesGeneral Output Control symbolic linksFile and Directory Selection
symbolic linksFile and Directory Selection symbolic linksFile and Directory
Selection

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T tab-aligned content linesOutput Line Prefix Control TERM environment
variableEnvironment Variables translation of message languageEnvironment
Variables

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U unspecified behavior in regular expressionsProblematic Expressions upper
character classCharacter Classes and Bracket Expressions upper-case
lettersCharacter Classes and Bracket Expressions usage summary, printingGeneric
Program Information usage, examplesUsage using grep, Q&AUsage

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V variants of grepgrep Programs version, printingGeneric Program Information

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W whitespace charactersCharacter Classes and Bracket Expressions with filename
prefixOutput Line Prefix Control

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X xdigit character classCharacter Classes and Bracket Expressions xdigit
classCharacter Classes and Bracket Expressions

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Z zero-terminated file namesOutput Line Prefix Control zero-terminated
linesOther Options

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