Linux下文本搜索工具grep命令使用入门

时间：2014-04-29 13:34:21 收藏：0 阅读：587

grep命令入门

如果想通过使用grep命令来实现理想化的文本搜索，对正则表达式的了解是比不可少的。文献1对正则表达式语法做了一个简单的介绍，文献2提供了一个简单的入门。码农也可以自己google一下其他的参考资料。下面就grep命令的使用做个入门级的介绍。

1.1 grep命令的变种

linux下除了grep命令可以完成文本搜索外，还存在egrep，fgrep，rgrep三个命令。这三个命令都是由grep加上一些控制参数演变而来，如egrep=grep -E, fgrep=grep -F, rgrep=grep -r(后面会介绍这些参数)，所以以码农只需关注于grep命令即可。

1.2 grep命令的使用格式

（1）grep [OPTIONS] PATTERN [FILE...]

（2）grep [OPTIONS] [-e PATTERN | -f PATTERN] [FILE...]

示例：

grep hello hello.c //查找hello.c文件中含有hello的代码行
grep -e hello -e world hello.c //查找文件中含有hello或world的代码行
grep -f hello hello.c //从文件hello中读取搜索关键词，然后在文件hello.c中查找此关键词

1.3 grep命令的参数选择

匹配选择：

-E, --extended-regexp 将PATTERN作为扩展的正则表达式解释，如grep -E ‘^\s‘ hello.c // 搜索hello.c文件中以空白字符开始的行，相当于egrep ‘^\s‘ hello.c

-F, --fixed-strings 将PATTERN解释为固定的字符串，不作正则表达式分析。grep -F ‘main()‘ hello.c // 等同于fgrep ‘main()‘ hello.c

匹配控制：

-e PATTERN, --regexp=PATTERN 指定pattern为搜索正则表达式，可以通过使用这个选项来指定多个搜索模式，或者用来制定以横线-开始的搜索模式，如grep -e hello -e world hello.c //搜索含有hello或者world的代码行

-f file, --file=FILE 指定从文件FILE中读取搜索模式，一行一个搜索模式

-i, --ignore-case模式匹配时忽略大小写

-v, --invert-math 反向匹配，选择那些不含有匹配模式的行，如grep -e -v hello hello.c //输出那些不含有hello的代码行

-x, --line-regexp 将一整行文本与搜索模式PATTERN进行匹配，输出匹配行，如grep -E ‘^\s*$‘ -x hello.c //查找空白行

输出控制：

-c, --count代替输出匹配的文本行，还是输出匹配的次数

-m NUM, --max-count=NUM 在一个文件中匹配次数超过NUM后，就停止搜索匹配

-o, --only-matching 代替输出匹配的文本行，而是仅仅输出匹配的部分文本

-n, --line-number 在输出的匹配文本行前面添加行号，这个行号指的原文件中的文本所在行号

其它：

-R, -r, --recursive 递归地从目录下的每个文件搜索匹配PATTERN，如grep -r hello . -n 10 //在当前目录下的每个文件（包括子目录中的文件）搜索单词hello，当在一个文件中匹配次数超过10次时，立即退出当前文件，对下一个文件进行搜索匹配

-d ACTION, --devices=ACTION 如果搜索文件是一个目录，使用ACTION来处理它。ACTION是read时，将目录文件当作普通文件；是skip时，将目录文件跳过；是recurse时，则递归地读取目录下的所有文件；grep hello -d recurse . //相当于grep hello -r .

注：rgrep hello . = grep hello -r . = grep hello -d recurse .

参考文献：

文献1：http://msdn.microsoft.com/zh-cn/library/ae5bf541(v=vs.90).aspx

文献2：http://regex.bastardsbook.com/

文献3：http://my.oschina.net/wenhaowu/blog/219956

附上手册：

GREP(1) GREP(1)

NAME
grep, egrep, fgrep, rgrep - print lines matching a pattern

SYNOPSIS
grep [OPTIONS] PATTERN [FILE...]
grep [OPTIONS] [-e PATTERN | -f FILE] [FILE...]

DESCRIPTION
grep searches the named input FILEs (or standard input if no files are
named, or if a single hyphen-minus (-) is given as file name) for lines
containing a match to the given PATTERN. By default, grep prints the
matching lines.

In addition, three variant programs egrep, fgrep and rgrep are
available. egrep is the same as grep -E. fgrep is the same as
grep -F. rgrep is the same as grep -r. Direct invocation as either
egrep or fgrep is deprecated, but is provided to allow historical
applications that rely on them to run unmodified.

OPTIONS
Generic Program Information
--help Print a usage message briefly summarizing these 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 (see
below).

Matcher Selection
-E, --extended-regexp
Interpret PATTERN as an extended regular expression (ERE, see
below). (-E is specified by POSIX.)

-F, --fixed-strings
Interpret PATTERN as a list of fixed strings, separated by
newlines, any of which is to be matched. (-F is specified by
POSIX.)

-G, --basic-regexp
Interpret PATTERN as a basic regular expression (BRE, see
below). This is the default.

-P, --perl-regexp
Interpret PATTERN as a Perl regular expression (PCRE, see
below). This is highly experimental and grep -P may warn of
unimplemented features.

Matching Control
-e PATTERN, --regexp=PATTERN
Use PATTERN as the pattern. This can be used to specify
multiple search patterns, or to protect a pattern beginning with
a hyphen (-). (-e is specified by POSIX.)

-f FILE, --file=FILE
Obtain patterns from FILE, one per line. The empty file
contains zero patterns, and therefore matches nothing. (-f is
specified by POSIX.)

-i, --ignore-case
Ignore case distinctions in both the PATTERN and the input
files. (-i is specified by POSIX.)

-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.

-x, --line-regexp
Select only those matches that exactly match the whole line.
(-x is specified by POSIX.)

-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. (-c is specified by POSIX.)

--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. (-l is specified by
POSIX.)

-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.
(-q is specified by POSIX.)

-s, --no-messages
Suppress error messages about nonexistent or unreadable files.
Portability note: unlike GNU grep, 7th Edition Unix grep did not
conform to POSIX, because it lacked -q and its -s option behaved
like GNU grep‘s -q option. USG-style grep also lacked -q but
its -s option behaved like GNU grep. Portable shell scripts
should avoid both -q and -s and should redirect standard and
error output to /dev/null instead. (-s is specified by POSIX.)

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 is especially useful when
implementing tools like zgrep, e.g., gzip -cd foo.gz | grep
--label=foo -H something. See also the -H option.

-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. 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 the first few bytes of a file indicate that the file contains
binary data, assume that the file is of type TYPE. By default,
TYPE is binary, and grep normally outputs either a one-line
message saying that a binary file matches, or no message if
there is no match. If TYPE is without-match, grep assumes that
a binary 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. Warning: grep
--binary-files=text 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.

-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, which means that directories are read
just as if they were ordinary files. If ACTION is skip,
directories are silently skipped. If ACTION is recurse, grep
reads all files under each directory, recursively; this is
equivalent to the -r option.

--exclude=GLOB
Skip files whose base name matches GLOB (using wildcard
matching). A file-name glob 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=DIR
Exclude directories matching the pattern DIR from recursive
searches.

-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).

-R, -r, --recursive
Read all files under each directory, recursively; this is
equivalent to the -d recurse option.

Other Options
--line-buffered
Use line buffering on output. This can cause a performance
penalty.

--mmap If possible, use the mmap(2) system call to read input, instead
of the default read(2) system call. In some situations, --mmap
yields better performance. However, --mmap can cause undefined
behavior (including core dumps) if an input file shrinks while
grep is operating, or if an I/O error occurs.

-U, --binary
Treat the file(s) as binary. By default, under MS-DOS and MS-
Windows, grep guesses the file type by looking at the contents
of the first 32KB read from the file. 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 the input as a set 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” (PRCE). 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 regular expressions give additional functionality,
and are documented in pcresyntax(3) and pcrepattern(3), but may not be
available on every 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.

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.
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:], [: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.
{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 $.

Traditional egrep did not support the { meta-character, and some egrep
implementations support \{ instead, so portable scripts should avoid {
in grep -E patterns and should use [{] to match a literal {.

GNU grep -E attempts to support traditional usage by assuming that { is
not special if it would be the start of an invalid interval
specification. For example, the command grep -E ‘{1‘ searches for the
two-character string {1 instead of reporting a syntax error in the
regular expression. POSIX.2 allows this behavior as an extension, but
portable scripts should avoid it.

ENVIRONMENT VARIABLES
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).

GREP_OPTIONS
This variable specifies default options to be placed in front of
any explicit options. For example, if GREP_OPTIONS is
‘--binary-files=without-match --directories=skip‘, grep behaves
as if the two options --binary-files=without-match and
--directories=skip had been specified before any explicit
options. Option specifications are separated by whitespace. A
backslash escapes the next character, so it can be used to
specify an option containing whitespace or a backslash.

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.

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.2 requires; otherwise, grep
behaves more like other GNU programs. POSIX.2 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.2 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.

EXIT STATUS
The exit status is 0 if selected lines are found, and 1 if not found.
If an error occurred the exit status is 2. (Note: POSIX error handling
code should check for ‘2‘ or greater.)

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 <bug-grep@gnu.org>, a mailing list whose web page
is <http://lists.gnu.org/mailman/listinfo/bug-grep>. grep‘s Savannah
bug tracker is located at <http://savannah.gnu.org/bugs/?group=grep>.

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.

SEE ALSO
Regular Manual Pages
awk(1), cmp(1), diff(1), find(1), gzip(1), perl(1), sed(1), sort(1),
xargs(1), zgrep(1), mmap(2), read(2), pcre(3), pcresyntax(3),
pcrepattern(3), terminfo(5), glob(7), regex(7).

POSIX Programmer‘s Manual Page
grep(1p).

TeXinfo Documentation
The full documentation for grep is maintained as a TeXinfo manual. If
the info and grep programs are properly installed at your site, the
command

info grep

should give you access to the complete manual.

NOTES
GNU‘s not Unix, but Unix is a beast; its plural form is Unixen.

User Commands GNU grep 2.10 GREP(1)