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(grep.info)Regular Expressions

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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 expressions 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
characters, separated by a hyphen.

   Finally, certain named classes of characters are predefined, as
follows.  Their interpretation depends on the `LC_CTYPE' locale; the
interpretation below is that of the POSIX locale, which is the default
if no `LC_CTYPE' locale is specified.

`[:alnum:]'
     Alphanumeric characters: `[:alpha:]' and `[:digit:]'.

`[:alpha:]'
     Alphabetic characters: `[:lower:]' and `[:upper:]'.

`[: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: `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: `! " # $ % & ' ( ) * + , - . / : ; < = > ?
     @ [ \ ] ^ _ ` { | } ~'.

`[:space:]'
     Space characters: tab, newline, vertical tab, form feed, carriage
     return, and space.

`[:upper:]'
     Upper-case letters: `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'.

   For example, `[[:alnum:]]' means `[0-9A-Za-z]', except the latter
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 will be 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 concatenating 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 subexpression 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 assuming 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.