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

     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 subexpressions.

     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

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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