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.TH REGEX 7 "7 Feb 1994"
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regex \- POSIX 1003.2 regular expressions
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Regular expressions (``RE''s),
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as defined in POSIX 1003.2, come in two forms:
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modern REs (roughly those of
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1003.2 calls these ``extended'' REs)
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and obsolete REs (roughly those of
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1003.2 ``basic'' REs).
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Obsolete REs mostly exist for backward compatibility in some old programs;
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they will be discussed at the end.
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1003.2 leaves some aspects of RE syntax and semantics open;
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`\(dg' marks decisions on these aspects that
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may not be fully portable to other 1003.2 implementations.
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A (modern) RE is one\(dg or more non-empty\(dg \fIbranches\fR,
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It matches anything that matches one of the branches.
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A branch is one\(dg or more \fIpieces\fR, concatenated.
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It matches a match for the first, followed by a match for the second, etc.
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A piece is an \fIatom\fR possibly followed
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by a single\(dg `*', `+', `?', or \fIbound\fR.
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An atom followed by `*' matches a sequence of 0 or more matches of the atom.
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An atom followed by `+' matches a sequence of 1 or more matches of the atom.
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An atom followed by `?' matches a sequence of 0 or 1 matches of the atom.
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A \fIbound\fR is `{' followed by an unsigned decimal integer,
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possibly followed by `,'
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possibly followed by another unsigned decimal integer,
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always followed by `}'.
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The integers must lie between 0 and RE_DUP_MAX (255\(dg) inclusive,
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and if there are two of them, the first may not exceed the second.
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An atom followed by a bound containing one integer \fIi\fR
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a sequence of exactly \fIi\fR matches of the atom.
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An atom followed by a bound
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containing one integer \fIi\fR and a comma matches
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a sequence of \fIi\fR or more matches of the atom.
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An atom followed by a bound
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containing two integers \fIi\fR and \fIj\fR matches
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a sequence of \fIi\fR through \fIj\fR (inclusive) matches of the atom.
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An atom is a regular expression enclosed in `()' (matching a match for the
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an empty set of `()' (matching the null string)\(dg,
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a \fIbracket expression\fR (see below), `.'
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(matching any single character), `^' (matching the null string at the
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beginning of a line), `$' (matching the null string at the
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end of a line), a `\e' followed by one of the characters
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(matching that character taken as an ordinary character),
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a `\e' followed by any other character\(dg
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(matching that character taken as an ordinary character,
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as if the `\e' had not been present\(dg),
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or a single character with no other significance (matching that character).
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A `{' followed by a character other than a digit is an ordinary
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character, not the beginning of a bound\(dg.
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It is illegal to end an RE with `\e'.
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A \fIbracket expression\fR is a list of characters enclosed in `[]'.
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It normally matches any single character from the list (but see below).
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If the list begins with `^',
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it matches any single character
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(but see below) \fInot\fR from the rest of the list.
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If two characters in the list are separated by `\-', this is shorthand
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for the full \fIrange\fR of characters between those two (inclusive) in the
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e.g. `[0-9]' in ASCII matches any decimal digit.
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It is illegal\(dg for two ranges to share an
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endpoint, e.g. `a-c-e'.
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Ranges are very collating-sequence-dependent,
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and portable programs should avoid relying on them.
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To include a literal `]' in the list, make it the first character
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(following a possible `^').
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To include a literal `\-', make it the first or last character,
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or the second endpoint of a range.
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To use a literal `\-' as the first endpoint of a range,
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enclose it in `[.' and `.]' to make it a collating element (see below).
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With the exception of these and some combinations using `[' (see next
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paragraphs), all other special characters, including `\e', lose their
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special significance within a bracket expression.
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Within a bracket expression, a collating element (a character,
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a multi-character sequence that collates as if it were a single character,
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or a collating-sequence name for either)
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enclosed in `[.' and `.]' stands for the
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sequence of characters of that collating element.
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The sequence is a single element of the bracket expression's list.
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A bracket expression containing a multi-character collating element
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can thus match more than one character,
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e.g. if the collating sequence includes a `ch' collating element,
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then the RE `[[.ch.]]*c' matches the first five characters
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Within a bracket expression, a collating element enclosed in `[=' and
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`=]' is an equivalence class, standing for the sequences of characters
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of all collating elements equivalent to that one, including itself.
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(If there are no other equivalent collating elements,
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the treatment is as if the enclosing delimiters were `[.' and `.]'.)
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For example, if o and \o'o^' are the members of an equivalence class,
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then `[[=o=]]', `[[=\o'o^'=]]', and `[o\o'o^']' are all synonymous.
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An equivalence class may not\(dg be an endpoint
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Within a bracket expression, the name of a \fIcharacter class\fR enclosed
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in `[:' and `:]' stands for the list of all characters belonging to that
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Standard character class names are:
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These stand for the character classes defined in
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A locale may provide others.
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A character class may not be used as an endpoint of a range.
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There are two special cases\(dg of bracket expressions:
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the bracket expressions `[[:<:]]' and `[[:>:]]' match the null string at
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the beginning and end of a word respectively.
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A word is defined as a sequence of
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which is neither preceded nor followed by
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A word character is an
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character (as defined by
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This is an extension,
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compatible with but not specified by POSIX 1003.2,
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and should be used with
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caution in software intended to be portable to other systems.
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In the event that an RE could match more than one substring of a given
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the RE matches the one starting earliest in the string.
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If the RE could match more than one substring starting at that point,
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it matches the longest.
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Subexpressions also match the longest possible substrings, subject to
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the constraint that the whole match be as long as possible,
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with subexpressions starting earlier in the RE taking priority over
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Note that higher-level subexpressions thus take priority over
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their lower-level component subexpressions.
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Match lengths are measured in characters, not collating elements.
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A null string is considered longer than no match at all.
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`bb*' matches the three middle characters of `abbbc',
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`(wee|week)(knights|nights)' matches all ten characters of `weeknights',
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when `(.*).*' is matched against `abc' the parenthesized subexpression
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matches all three characters, and
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when `(a*)*' is matched against `bc' both the whole RE and the parenthesized
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subexpression match the null string.
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If case-independent matching is specified,
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the effect is much as if all case distinctions had vanished from the
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When an alphabetic that exists in multiple cases appears as an
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ordinary character outside a bracket expression, it is effectively
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transformed into a bracket expression containing both cases,
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e.g. `x' becomes `[xX]'.
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When it appears inside a bracket expression, all case counterparts
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of it are added to the bracket expression, so that (e.g.) `[x]'
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becomes `[xX]' and `[^x]' becomes `[^xX]'.
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No particular limit is imposed on the length of REs\(dg.
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Programs intended to be portable should not employ REs longer
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as an implementation can refuse to accept such REs and remain
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Obsolete (``basic'') regular expressions differ in several respects.
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`|', `+', and `?' are ordinary characters and there is no equivalent
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for their functionality.
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The delimiters for bounds are `\e{' and `\e}',
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with `{' and `}' by themselves ordinary characters.
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The parentheses for nested subexpressions are `\e(' and `\e)',
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with `(' and `)' by themselves ordinary characters.
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`^' is an ordinary character except at the beginning of the
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RE or\(dg the beginning of a parenthesized subexpression,
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`$' is an ordinary character except at the end of the
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RE or\(dg the end of a parenthesized subexpression,
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and `*' is an ordinary character if it appears at the beginning of the
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RE or the beginning of a parenthesized subexpression
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(after a possible leading `^').
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Finally, there is one new type of atom, a \fIback reference\fR:
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`\e' followed by a non-zero decimal digit \fId\fR
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matches the same sequence of characters
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matched by the \fId\fRth parenthesized subexpression
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(numbering subexpressions by the positions of their opening parentheses,
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so that (e.g.) `\e([bc]\e)\e1' matches `bb' or `cc' but not `bc'.
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POSIX 1003.2, section 2.8 (Regular Expression Notation).
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Having two kinds of REs is a botch.
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The current 1003.2 spec says that `)' is an ordinary character in
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the absence of an unmatched `(';
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this was an unintentional result of a wording error,
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and change is likely.
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Back references are a dreadful botch,
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posing major problems for efficient implementations.
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They are also somewhat vaguely defined
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`a\e(\e(b\e)*\e2\e)*d' match `abbbd'?).
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1003.2's specification of case-independent matching is vague.
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The ``one case implies all cases'' definition given above
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is current consensus among implementors as to the right interpretation.
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The syntax for word boundaries is incredibly ugly.
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regex/regex.7
