regex − POSIX.2 regular expressions

Regular expressions ("RE"s), as defined in POSIX.2, come in two forms: modern
REs (roughly those of POSIX.2 calls these "extended" REs) and obsolete REs
(roughly those of POSIX.2 "basic" REs).  Obsolete REs mostly exist for backward
compatibility in some old programs; they will be discussed at the end.  POSIX.2
leaves some aspects of RE syntax and semantics open; "(!)" marks decisions on
these aspects that may not be fully portable to other POSIX.2 implementations.

     A (modern) RE is one(!) or more nonempty(!) branches, separated by '|'.  It
matches anything that matches one of the branches.

     A branch is one(!) or more pieces, concatenated.  It matches a match for
the first, followed by a match for the second, and so on.

     A piece is an atom possibly followed by a single(!) '*', '+', '?', or
bound.  An atom followed by '*' matches a sequence of 0 or more matches of the
atom.  An atom followed by '+' matches a sequence of 1 or more matches of the
atom.  An atom followed by '?' matches a sequence of 0 or 1 matches of the atom.

     A bound is '{' followed by an unsigned decimal integer, possibly followed
by ',' possibly followed by another unsigned decimal integer, always followed by
'}'.  The integers must lie between 0 and RE_DUP_MAX (255(!)) inclusive, and if
there are two of them, the first may not exceed the second.  An atom followed by
a bound containing one integer i and no comma matches a sequence of exactly i
matches of the atom.  An atom followed by a bound containing one integer i and a
comma matches a sequence of i or more matches of the atom.  An atom followed by
a bound containing two integers i and j matches a sequence of i through j
(inclusive) matches of the atom.

     An atom is a regular expression enclosed in "()" (matching a match for the
regular expression), an empty set of "()" (matching the null string)(!), a
bracket expression (see below), '.' (matching any single character), '^'
(matching the null string at the beginning of a line), '$' (matching the null
string at the end of a line), a '\' followed by one of the characters
"^.[$()|*+?{\" (matching that character taken as an ordinary character), a '\'
followed by any other character(!)  (matching that character taken as an
ordinary character, as if the '\' had not been present(!)), or a single
character with no other significance (matching that character).  A '{' followed
by a character other than a digit is an ordinary character, not the beginning of
a bound(!).  It is illegal to end an RE with '\'.

     A bracket expression is a list of characters enclosed in "[]".  It normally
matches any single character from the list (but see below).  If the list begins
with '^', it matches any single character (but see below) not from the rest of
the list.  If two characters in the list are separated by '−', this is shorthand
for the full range of characters between those two (inclusive) in the collating
sequence, for example, "[0−9]" in ASCII matches any decimal digit.  It is
illegal(!) for two ranges to share an endpoint, for example, "a−c−e".  Ranges
are very collating‐sequence‐dependent, and portable programs should avoid
relying on them.

     To include a literal ']' in the list, make it the first character
(following a possible '^').  To include a literal '−', make it the first or last


character, or the second endpoint of a range.  To use a literal '−' as the first
endpoint of a range, enclose it in "[." and ".]"  to make it a collating element
(see below).  With the exception of these and some combinations using '[' (see
next paragraphs), all other special characters, including '\', lose their
special significance within a bracket expression.

     Within a bracket expression, a collating element (a character, a
multicharacter sequence that collates as if it were a single character, or a
collating‐sequence name for either) enclosed in "[." and ".]" stands for the
sequence of characters of that collating element.  The sequence is a single
element of the bracket expression’s list.  A bracket expression containing a
multicharacter collating element can thus match more than one character, for
example, if the collating sequence includes a "ch" collating element, then the
RE "[[.ch.]]*c" matches the first five characters of "chchcc".

     Within a bracket expression, a collating element enclosed in "[=" and "=]"
is an equivalence class, standing for the sequences of characters of all
collating elements equivalent to that one, including itself.  (If there are no
other equivalent collating elements, the treatment is as if the enclosing
delimiters were "[." and ".]".)  For example, if o and ^ are the members of an
equivalence class, then "[[=o=]]", "[[=^=]]", and "[o^]" are all synonymous.  An
equivalence class may not(!) be an endpoint of a range.

     Within a bracket expression, the name of a character class enclosed in "[:"
and ":]" stands for the list of all characters belonging to that class.
Standard character class names are:

     alnum   digit   punct
     alpha   graph   space
     blank   lower   upper
     cntrl   print   xdigit

     These stand for the character classes defined in A locale may provide
others.  A character class may not be used as an endpoint of a range.

     In the event that an RE could match more than one substring of a given
string, the RE matches the one starting earliest in the string.  If the RE could
match more than one substring starting at that point, it matches the longest.
Subexpressions also match the longest possible substrings, subject to the
constraint that the whole match be as long as possible, with subexpressions
starting earlier in the RE taking priority over ones starting later.  Note that
higher‐level subexpressions thus take priority over their lower‐level component

     Match lengths are measured in characters, not collating elements.  A null
string is considered longer than no match at all.  For example, "bb*" matches
the three middle characters of "abbbc", "(wee|week)(knights|nights)" matches all
ten characters of "weeknights", when "(.*).*" is matched against "abc" the
parenthesized subexpression matches all three characters, and when "(a*)*" is
matched against "bc" both the whole RE and the parenthesized subexpression match
the null string.


     If case‐independent matching is specified, the effect is much as if all
case distinctions had vanished from the alphabet.  When an alphabetic that
exists in multiple cases appears as an ordinary character outside a bracket
expression, it is effectively transformed into a bracket expression containing
both cases, for example, 'x' becomes "[xX]".  When it appears inside a bracket
expression, all case counterparts of it are added to the bracket expression, so
that, for example, "[x]" becomes "[xX]" and "[^x]" becomes "[^xX]".

     No particular limit is imposed on the length of REs(!).  Programs intended
to be portable should not employ REs longer than 256 bytes, as an implementation
can refuse to accept such REs and remain POSIX‐compliant.

     Obsolete ("basic") regular expressions differ in several respects.  '|',
'+', and '?' are ordinary characters and there is no equivalent for their
functionality.  The delimiters for bounds are "\{" and "\}", with '{' and '}' by
themselves ordinary characters.  The parentheses for nested subexpressions are
"\(" and "\)", with '(' and ')' by themselves ordinary characters.  '^' is an
ordinary character except at the beginning of the RE or(!) the beginning of a
parenthesized subexpression, '$' is an ordinary character except at the end of
the RE or(!) the end of a parenthesized subexpression, and '*' is an ordinary
character if it appears at the beginning of the RE or the beginning of a
parenthesized subexpression (after a possible leading '^').

     Finally, there is one new type of atom, a back reference: '\' followed by a
nonzero decimal digit d matches the same sequence of characters matched by the
dth parenthesized subexpression (numbering subexpressions by the positions of
their opening parentheses, left to right), so that, for example, "\([bc]\)\1"
matches "bb" or "cc" but not "bc".

Having two kinds of REs is a botch.

     The current POSIX.2 spec says that ')' is an ordinary character in the
absence of an unmatched '('; this was an unintentional result of a wording
error, and change is likely.  Avoid relying on it.

     Back references are a dreadful botch, posing major problems for efficient
implementations.  They are also somewhat vaguely defined (does "a\(\(b\)*\2\)*d"
match "abbbd"?).  Avoid using them.

     POSIX.2’s specification of case‐independent matching is vague.  The "one
case implies all cases" definition given above is current consensus among
implementors as to the right interpretation.

This page was taken from Henry Spencer’s regex package.

     POSIX.2, section 2.8 (Regular Expression Notation).

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