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.
Character Classes and Bracket Expressions
A bracket expression
is a list of characters enclosed by [
. 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 
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.
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 \<
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:]]
is a synonym for [^_[:alnum:]]
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.
- The preceding item is matched exactly n times.
- The preceding item is matched n or more times.
- The preceding item is matched at most m times. This is a GNU extension.
- 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 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 alternate expression.
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 n
th 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 \)
Regular Manual Pages
awk(1), cmp(1), diff(1), find(1), gzip(1), perl(1), sed(1), sort(1), xargs(1), zgrep(1), read(2), pcre(3), pcresyntax(3), pcrepattern(3), terminfo(5), glob(7), regex(7).
POSIX Programmer's Manual Page
A complete manual <URL: http://www.gnu.org/software/grep/manual/ > is available. If the info
programs are properly installed at your site, the command
- info grep
should give you access to the complete manual.