Grep
Last updated on
What is GREP
GREP stands for Global Regular Expresion Print. While the name does sound like grep would actually print out regular expressions, or regex for short, but it actually prints out matches to those regular expressions.
Grep a powerful yet rarely understood tool that is used to match patterns.
According to grep’s built-in --help function, this is the structure of grep.
Usage: grep [OPTION]... PATTERNS [FILE]...
Search for PATTERNS in each FILE.
Example: grep -i 'hello world' menu.h main.c
PATTERNS can contain multiple patterns separated by newlines.
Pattern selection and interpretation:
-E, --extended-regexp PATTERNS are extended regular expressions
-F, --fixed-strings PATTERNS are strings
-G, --basic-regexp PATTERNS are basic regular expressions
-P, --perl-regexp PATTERNS are Perl regular expressions
-e, --regexp=PATTERNS use PATTERNS for matching
-f, --file=FILE take PATTERNS from FILE
-i, --ignore-case ignore case distinctions in patterns and data
--no-ignore-case do not ignore case distinctions (default)
-w, --word-regexp match only whole words
-x, --line-regexp match only whole lines
-z, --null-data a data line ends in 0 byte, not newline
Miscellaneous:
-s, --no-messages suppress error messages
-v, --invert-match select non-matching lines
-V, --version display version information and exit
--help display this help text and exit
Output control:
-m, --max-count=NUM stop after NUM selected lines
-b, --byte-offset print the byte offset with output lines
-n, --line-number print line number with output lines
--line-buffered flush output on every line
-H, --with-filename print file name with output lines
-h, --no-filename suppress the file name prefix on output
--label=LABEL use LABEL as the standard input file name prefix
-o, --only-matching show only nonempty parts of lines that match
-q, --quiet, --silent suppress all normal output
--binary-files=TYPE assume that binary files are TYPE;
TYPE is 'binary', 'text', or 'without-match'
-a, --text equivalent to --binary-files=text
-I equivalent to --binary-files=without-match
-d, --directories=ACTION how to handle directories;
ACTION is 'read', 'recurse', or 'skip'
-D, --devices=ACTION how to handle devices, FIFOs and sockets;
ACTION is 'read' or 'skip'
-r, --recursive like --directories=recurse
-R, --dereference-recursive likewise, but follow all symlinks
--include=GLOB search only files that match GLOB (a file pattern)
--exclude=GLOB skip files that match GLOB
--exclude-from=FILE skip files that match any file pattern from FILE
--exclude-dir=GLOB skip directories that match GLOB
-L, --files-without-match print only names of FILEs with no selected lines
-l, --files-with-matches print only names of FILEs with selected lines
-c, --count print only a count of selected lines per FILE
-T, --initial-tab make tabs line up (if needed)
-Z, --null print 0 byte after FILE name
Context control:
-B, --before-context=NUM print NUM lines of leading context
-A, --after-context=NUM print NUM lines of trailing context
-C, --context=NUM print NUM lines of output context
-NUM same as --context=NUM
--color[=WHEN],
--colour[=WHEN] use markers to highlight the matching strings;
WHEN is 'always', 'never', or 'auto'
-U, --binary do not strip CR characters at EOL (MSDOS/Windows)
When FILE is '-', read standard input. With no FILE, read '.' if
recursive, '-' otherwise. With fewer than two FILEs, assume -h.
Exit status is 0 if any line is selected, 1 otherwise;
if any error occurs and -q is not given, the exit status is 2.
Report bugs to: bug-grep@gnu.org
GNU grep home page: <http://www.gnu.org/software/grep/>
General help using GNU software: <https://www.gnu.org/gethelp/>So what if you are a man page user? Well it might look more like this:
GREP(1) User Commands GREP(1)
NAME
grep, egrep, fgrep, rgrep - print lines that match patterns
SYNOPSIS
grep [OPTION...] PATTERNS [FILE...]
grep [OPTION...] -e PATTERNS ... [FILE...]
grep [OPTION...] -f PATTERN_FILE ... [FILE...]
DESCRIPTION
grep searches for PATTERNS in each FILE. PATTERNS is one or more patterns separated by newline characters,
and grep prints each line that matches a pattern. Typically PATTERNS should be quoted when grep is used in a
shell command.
A FILE of “-” stands for standard input. If no FILE is given, recursive searches examine the working
directory, and nonrecursive searches read standard input.
In addition, the variant programs egrep, fgrep and rgrep are the same as grep -E, grep -F, and grep -r,
respectively. These variants are deprecated, but are provided for backward compatibility.
OPTIONS
Generic Program Information
--help Output a usage message and exit.
-V, --version
Output the version number of grep and exit.
Pattern Syntax
-E, --extended-regexp
Interpret PATTERNS as extended regular expressions (EREs, see below).
-F, --fixed-strings
Interpret PATTERNS as fixed strings, not regular expressions.
-G, --basic-regexp
Interpret PATTERNS as basic regular expressions (BREs, see below). This is the default.
-P, --perl-regexp
Interpret PATTERNS as Perl-compatible regular expressions (PCREs). This option is experimental when
combined with the -z (--null-data) option, and grep -P may warn of unimplemented features.
Matching Control
-e PATTERNS, --regexp=PATTERNS
Use PATTERNS as the patterns. If this option is used multiple times or is combined with the -f
(--file) option, search for all patterns given. This option can be used to protect a pattern beginning
with “-”.
-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. The empty file contains zero patterns, and
therefore matches nothing.
-i, --ignore-case
Ignore case distinctions in patterns and input data, so that characters that differ only in case match
each other.
--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, to cancel its effects because the two options
override each other.
-v, --invert-match
Invert the sense of matching, to select non-matching lines.
-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.
-x, --line-regexp
Select only those matches that exactly match the whole line. For a regular expression pattern, this is
like parenthesizing the pattern and then surrounding it with ^ and $.
-y Obsolete synonym for -i.
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 (see below), count non-matching lines.
--color[=WHEN], --colour[=WHEN]
Surround the 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. The
deprecated environment variable GREP_COLOR is still supported, but its setting does not have priority.
WHEN is never, always, or auto.
-L, --files-without-match
Suppress normal output; instead print the name of each input file from which no output would normally
have been printed. The scanning will stop on the first match.
-l, --files-with-matches
Suppress normal output; instead print the name of each input file from which output would normally have
been printed. The scanning will stop on the first match.
-m NUM, --max-count=NUM
Stop reading a file after NUM matching lines. If the input is standard input from a regular file, and
NUM matching lines are output, grep ensures that the standard input is positioned to just after the
last matching line before exiting, regardless of the presence of trailing context lines. This enables
a calling process to resume a search. When grep stops after NUM matching 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 a matching line, with each such part on a separate output
line.
-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.
-s, --no-messages
Suppress error messages about nonexistent or unreadable files.
Output Line Prefix Control
-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'. See also the -H option.
-n, --line-number
Prefix each line of output with the 1-based line number within its input file.
-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. In order to improve the probability that lines from a single file will all start at the same
column, this also causes the line number and byte offset (if present) to be printed in a minimum size
field width.
-u, --unix-byte-offsets
Report Unix-style byte offsets. This switch causes grep to report byte offsets as if the file were a
Unix-style text file, i.e., with CR characters stripped off. This will produce results identical to
running grep on a Unix machine. This option has no effect unless -b option is also used; it has no
effect on platforms other than MS-DOS and MS-Windows.
-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.
Context Line Control
-A NUM, --after-context=NUM
Print NUM lines of trailing context after matching lines. Places a line containing a group separator
(--) between contiguous groups of matches. With the -o or --only-matching option, this has no effect
and a warning is given.
-B NUM, --before-context=NUM
Print NUM lines of leading context before matching lines. Places a line containing a group separator
(--) between contiguous groups of matches. With the -o or --only-matching option, this has no effect
and a warning is given.
-C NUM, -NUM, --context=NUM
Print NUM lines of output context. Places a line containing a group separator (--) between contiguous
groups of matches. With the -o or --only-matching option, this has no effect and a warning is given.
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, or null input bytes when the -z option is not given.
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 one-line message 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 a binary file 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 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 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. By default, ACTION is read,
which means that devices are read just as if they were ordinary files. If ACTION is skip, devices are
silently skipped.
-d ACTION, --directories=ACTION
If an input file is a directory, use ACTION to process it. By default, ACTION is read, i.e., read
directories just as if they were ordinary files. If ACTION is skip, silently skip directories. If
ACTION is recurse, read all files under each directory, recursively, following symbolic links only if
they are on the command line. 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 base name matches any of the file-name globs 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 base 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
Read all files under each directory, recursively, following symbolic links only if they are on the
command line. Note that if no file operand is given, grep searches the working directory. This is
equivalent to the -d recurse option.
-R, --dereference-recursive
Read all files under each directory, recursively. Follow all symbolic links, unlike -r.
Other Options
--line-buffered
Use line buffering on output. This can cause a performance penalty.
-U, --binary
Treat the file(s) as binary. By default, under MS-DOS and MS-Windows, grep guesses whether a file is
text or binary as described for the --binary-files option. If grep decides the file is a text file, it
strips the CR characters from the original file contents (to make regular expressions with ^ and $ work
correctly). Specifying -U overrules this guesswork, causing all files to be read and passed to the
matching mechanism verbatim; if the file is a text file with CR/LF pairs at the end of each line, this
will cause some regular expressions to fail. This option has no effect on platforms other than MS-DOS
and MS-Windows.
-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.
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” (PCRE). In GNU grep there is no difference in available functionality between basic and extended
syntaxes. In other implementations, basic regular expressions are less powerful. The following description
applies to extended regular expressions; differences for basic regular expressions are summarized afterwards.
Perl-compatible regular expressions give additional functionality, and are documented in pcresyntax(3) and
pcrepattern(3), but work only if PCRE is available in the system.
The fundamental building blocks are the regular expressions that match a single character. Most characters,
including all letters and digits, are regular expressions that match themselves. Any meta-character with
special meaning may be quoted by preceding it with a backslash.
The period . matches any single character. It is unspecified whether it matches an encoding error.
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; it is
unspecified whether it matches an encoding error. For example, the regular expression [0123456789] matches
any single digit.
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, using the locale's collating sequence
and character set. For example, in the default C locale, [a-d] is equivalent to [abcd]. Many locales sort
characters in dictionary order, and in these locales [a-d] is typically not equivalent to [abcd]; it might be
equivalent to [aBbCcDd], for example. 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
names are self explanatory, and they are [:alnum:], [:alpha:], [:blank:], [:cntrl:], [:digit:], [:graph:],
[:lower:], [:print:], [:punct:], [:space:], [:upper:], and [:xdigit:]. For example, [[:alnum:]] means the
character class of numbers and letters in the current locale. In the C locale and ASCII character set
encoding, this is the same as [0-9A-Za-z]. (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.) Most
meta-characters lose their special meaning inside bracket expressions. To include a literal ] place it first
in the list. Similarly, to include a literal ^ place it anywhere but first. Finally, to include a literal -
place it last.
Anchoring
The caret ^ and the dollar sign $ are meta-characters that respectively match the empty string at the
beginning and end of a line.
The Backslash Character and Special Expressions
The symbols \< and \> respectively match the empty string at the beginning and end of a word. The symbol \b
matches the empty string at the edge of a word, and \B matches the empty string provided it's not at the edge
of a word. The symbol \w is a synonym for [_[:alnum:]] and \W is a synonym for [^_[:alnum:]].
Repetition
A regular expression may be followed by one of several repetition operators:
? The preceding item is optional and matched at most once.
* The preceding item will be matched zero or more times.
+ The preceding item will be 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.
Concatenation
Two regular expressions may be concatenated; the resulting regular expression matches any string formed by
concatenating two substrings that respectively match the concatenated expressions.
Alternation
Two regular expressions may be joined by the infix operator |; the resulting regular expression matches any
string matching either alternate expression.
Precedence
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.
Back-references and Subexpressions
The back-reference \n, where n is a single digit, matches the substring previously matched by the nth
parenthesized subexpression of the regular expression.
Basic vs Extended Regular Expressions
In basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; instead use
the backslashed versions \?, \+, \{, \|, \(, and \).
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 is used and a line is selected, the exit status is 0 even if an
error occurred.
ENVIRONMENT
The behavior of grep is affected by the following environment variables.
The locale for category LC_foo is specified by examining the three environment variables LC_ALL, LC_foo, 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_MESSAGES is set to pt_BR, then the Brazilian Portuguese locale is used for 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.
GREP_COLOR
This variable specifies the color used to highlight matched (non-empty) text. It is deprecated in
favor of GREP_COLORS, but still supported. The mt, ms, and mc capabilities of GREP_COLORS have
priority over it. It can only specify the color used to highlight the matching non-empty text in any
matching line (a selected line when the -v command-line option is omitted, or a context line when -v is
specified). The default is 01;31, which means a bold red foreground text on the terminal's default
background.
GREP_COLORS
Specifies the colors and other attributes used to highlight various parts of the output. Its value is
a colon-separated list of 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). 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).
rv 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 only used when the -v
command-line option is omitted.) The effect of the sl= (or cx= if rv) capability remains active
when this kicks in. 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 only used when the -v
command-line option is specified.) The effect of the cx= (or sl= if rv) capability remains
active when this kicks in. 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.
ne 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.
See the Select Graphic Rendition (SGR) section in the documentation of the text terminal that is used
for permitted values and their meaning 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.
LC_ALL, LC_COLLATE, LANG
These variables specify the locale for the LC_COLLATE category, which determines the collating sequence
used to interpret range expressions like [a-z].
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, that is,
whether text is encoded in UTF-8, ASCII, or some other encoding. In the C or POSIX locale, all
characters are encoded as a single byte and every byte is a valid character.
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. Also, POSIX requires that
unrecognized options be diagnosed as “illegal”, but since they are not really against the law the
default is to diagnose them as “invalid”. POSIXLY_CORRECT also disables _N_GNU_nonoption_argv_flags_,
described below.
_N_GNU_nonoption_argv_flags_
(Here N is grep's numeric process ID.) If the ith character of this environment variable's value is 1,
do not consider the ith operand of grep to be an option, even if it appears to be one. A shell can put
this variable in the environment for each command it runs, specifying which operands are the results of
file name wildcard expansion and therefore should not be treated as options. This behavior is
available only with the GNU C library, and only when POSIXLY_CORRECT is not set.
NOTES
This man page is maintained only fitfully; the full documentation is often more up-to-date.
COPYRIGHT
Copyright 1998-2000, 2002, 2005-2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO warranty; not even for
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
BUGS
Reporting Bugs
Email bug reports to the bug-reporting address ⟨bug-grep@gnu.org⟩. An email archive
⟨https://lists.gnu.org/mailman/listinfo/bug-grep⟩ and a bug tracker
⟨https://debbugs.gnu.org/cgi/pkgreport.cgi?package=grep⟩ are available.
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 are very slow, and may require exponential time.
EXAMPLE
The following example outputs the location and contents of any line containing “f” and ending in “.c”, within
all files in the current directory whose names contain “g” and end in “.h”. The -n option outputs line num‐
bers, the -- argument treats expansions of “*g*.h” starting with “-” as file names not options, 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$' *g*.h /dev/null
argmatch.h:1:/* definitions and prototypes for argmatch.c
The only line that matches is line 1 of argmatch.h. Note that the regular expression syntax used in the pat‐
tern differs from the globbing syntax that the shell uses to match file names.
SEE ALSO
Regular Manual Pages
awk(1), cmp(1), diff(1), find(1), perl(1), sed(1), sort(1), xargs(1), read(2), pcre(3), pcresyntax(3),
pcrepattern(3), terminfo(5), glob(7), regex(7).
Full Documentation
A complete manual ⟨https://www.gnu.org/software/grep/manual/⟩ is available. If the info and grep programs are
properly installed at your site, the command
info grep
should give you access to the complete manual.
GNU grep 3.6 2019-12-29 GREP(1)What is REGEX
Regex, or Regular Expression, is a way of writing search queries that look for specific phrases. A regex expression is a form of pattern matching. For the purpose of this article we will skip the regex tutorial.
Here are some basics for a recap.
Anchors
These are expressions that signify a position as a condition. Using one of these means that for there to be a match then you must find the particular phrase in the required position. For example, when we use ^ we are signifying that the phrase after it must be at the beginning of any match. Say ^c would be true for chris but would be false for back. In the first case we have the word chris which begins with a c. This provides a match and thus is true for the regex of ^c. Since back only contains a c at the end it does not satisfy the case of ^c.
| Expression | Description | Example |
|---|---|---|
| ^[exp] | Start | ^a - “Hola amigo” |
| \A[exp] | Start | \Aa - “Hola amigo” |
| [exp]$ | End | d$ - “Hello World” |
| [exp]\Z | End | d\Z - “Hello World” |
| \b[exp]\b | Word Boundary | \bme\b - “me not some” |
| \B[exp]\B | Not Word Boundary | \Bme\B - “Homer not me or some” |
Note: The use of [exp] is designed to represent a standalone expression. This should not be confused wtih the actual brackets notation.
Let’s look at those work boudaries. They are a bit strange but useful. If a word is a group of letters then a word boundary is the edge of that word. In other words, a letter next to a space would be a word boundary. /b is a word boundary. The side of the expression you apply /b to is the side of the expression that you want a boundary on. If both side have a boundary then you can safely say you are looking for an exact word match. This looks like /bword\b. If we want a phrase that ends with an expression then we can use [exp]\b.
TODO: add some examples of these being demonstrated in the console. Python or JS regex.