search input files for lines that match a given pattern.
Grep searches the named input FILEs (or standard input if no files are named, or the file name - is given) for lines containing a match to the given PATTERN. By default, grep prints the matching lines. In addition, two variant programs egrep and fgrep are available. Egrep is the same as grep -E. Fgrep is the same as grep -F.
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 two different versions of regular expression syntax: "basic" and "extended." In GNU grep, there
is no difference in available functionality using either
syntax. In other implementations, basic regular expres-
sions are less powerful. The following description
applies to extended regular expressions; differences for
basic regular expressions are summarized afterwards.
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 metacharacter with special
meaning may be quoted by preceding it with a backslash.
A list of characters enclosed by [ and ] 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. A range of characters may be specified by giving the first and last charac-
ters, separated by a hyphen. Finally, certain named
classes of characters are predefined:
[:alnum:] - Any digit or Alphanumeric
[:alpha:] - Any Alphanumeric
[:cntrl:] - octal codes 000 through 037, or `DEL' (octal 177)
[:digit:] - Any one of `0 1 2 3...7 8 9'
[:graph:] - Anything that is not a `[:alnum:]' or `[:punct:]'
[:lower:] - Any one of `a b c... x y z'
[:print:] - Any char from the `[:space:]' class, and any char not in the `[:graph:]' class.
[:punct:] - Any one of `! " # $ % & ' ( ) * + , - . / : ; < = > ? @ [ \ ] ^ _ ` { | } ~'
[:space:] - Any one of `CR FF HT NL VT SPACE'
[:upper:] - Any one of `A B C... X Y Z'
[:xdigit:] - Hex: `a b c d e f A B C D E F 0 1 2 3 4 5 6 7 8 9'
For example [[:alnum:]] means [0-9A-Za-z], except the latter
form depends upon the POSIX locale and the ASCII character
encoding, whereas the former is independent of locale and
character set. (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 list.)
Most metacharacters lose their special meaning inside
lists. 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.
The period . matches any single character. The symbol \w
is a synonym for [[:alnum:]] and \W is a synonym for
[^[:alnum]].
The caret ^ and the dollar sign $ are metacharacters that
respectively match the empty string at the beginning and
end of a line. 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.
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.
Two regular expressions may be concatenated; the resulting
regular expression matches any string formed by concate-
nating two substrings that respectively match the concatenated subexpressions.
Two regular expressions may be joined by the infix operator |; the resulting regular expression matches any string
matching either subexpression.
Repetition takes precedence over concatenation, which in
turn takes precedence over alternation. A whole subex-
pression may be enclosed in parentheses to override these
precedence rules.
The backreference \n, where n is a single digit, matches
the substring previously matched by the nth parenthesized
subexpression of the regular expression.
In basic regular expressions the metacharacters ?, +, {,
|, (, and ) lose their special meaning; instead use the
backslashed versions \?, \+, \{, \|, \(, and \).
Traditional egrep did not support the { metacharacter, and
some egrep implementations support \{ instead, so portable
scripts should avoid { in egrep patterns and should use
[{] to match a literal {.
GNU egrep attempts to support traditional usage by assum-
ing that { is not special if it would be the start of an
invalid interval specification. For example, the shell
command egrep '{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.
Related:
awk - Find and Replace text within file(s)
grep - Search file(s) for lines that match a given pattern
tr - Translate, squeeze, and/or delete characters
