find.info: posix-egrep regular expression syntax

Regular Expressions

posix-extended regular expression syntax

posix-basic regular expression syntax

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7.5.9 `posix-egrep' regular expression syntax
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The character `.' matches any single character except newline.

`+'
     indicates that the regular expression should match one or more
     occurrences of the previous atom or regexp.

`?'
     indicates that the regular expression should match zero or one
     occurrence of the previous atom or regexp.

`\+'
     matches a `+'

`\?'
     matches a `?'.

   Bracket expressions are used to match ranges of characters.  Bracket
expressions where the range is backward, for example `[z-a]', are
ignored.  Within square brackets, `\' is taken literally.  Character
classes are supported; for example `[[:digit:]]' will match a single
decimal digit.  Non-matching lists `[^...]' do not ever match newline.

   GNU extensions are supported:
  1. `\w' matches a character within a word

  2. `\W' matches a character which is not within a word

  3. `\<' matches the beginning of a word

  4. `\>' matches the end of a word

  5. `\b' matches a word boundary

  6. `\B' matches characters which are not a word boundary

  7. `\`' matches the beginning of the whole input

  8. `\'' matches the end of the whole input


   Grouping is performed with parentheses `()'.  A backslash followed
by a digit acts as a back-reference and matches the same thing as the
previous grouped expression indicated by that number.  For example `\2'
matches the second group expression.  The order of group expressions is
determined by the position of their opening parenthesis `('.

   The alternation operator is `|'.

   The characters `^' and `$' always represent the beginning and end of
a string respectively, except within square brackets.  Within brackets,
`^' can be used to invert the membership of the character class being
specified.

   The characters `*', `+' and `?' are special anywhere in a regular
expression.

   Intervals are specified by `{' and `}'.  Invalid intervals are
treated as literals, for example `a{1' is treated as `a\{1'

   The longest possible match is returned; this applies to the regular
expression as a whole and (subject to this constraint) to
subexpressions within groups.