1REGEX(7)                   Linux Programmer's Manual                  REGEX(7)
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NAME

6       regex - POSIX.2 regular expressions
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DESCRIPTION

9       Regular  expressions ("RE"s), as defined in POSIX.2, come in two forms:
10       modern REs (roughly those of egrep; POSIX.2 calls these "extended" REs)
11       and  obsolete REs (roughly those of ed(1); POSIX.2 "basic" REs).  Obso‐
12       lete REs mostly exist for backward compatibility in some old  programs;
13       they  will  be discussed at the end.  POSIX.2 leaves some aspects of RE
14       syntax and semantics open; "(!)" marks decisions on these aspects  that
15       may not be fully portable to other POSIX.2 implementations.
16
17       A (modern) RE is one(!) or more nonempty(!) branches, separated by '|'.
18       It matches anything that matches one of the branches.
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20       A branch is one(!) or more pieces, concatenated.  It  matches  a  match
21       for the first, followed by a match for the second, and so on.
22
23       A  piece  is an atom possibly followed by a single(!) '*', '+', '?', or
24       bound.  An atom followed by '*' matches a sequence of 0 or more matches
25       of  the  atom.  An atom followed by '+' matches a sequence of 1 or more
26       matches of the atom.  An atom followed by '?' matches a sequence  of  0
27       or 1 matches of the atom.
28
29       A  bound  is '{' followed by an unsigned decimal integer, possibly fol‐
30       lowed by ',' possibly followed by  another  unsigned  decimal  integer,
31       always followed by '}'.  The integers must lie between 0 and RE_DUP_MAX
32       (255(!)) inclusive, and if there are two of them,  the  first  may  not
33       exceed  the second.  An atom followed by a bound containing one integer
34       i and no comma matches a sequence of exactly i matches of the atom.  An
35       atom followed by a bound containing one integer i and a comma matches a
36       sequence of i or more matches of the atom.  An atom followed by a bound
37       containing  two  integers  i  and  j  matches a sequence of i through j
38       (inclusive) matches of the atom.
39
40       An atom is a regular expression enclosed in "()" (matching a match  for
41       the  regular  expression),  an  empty  set  of  "()" (matching the null
42       string)(!), a bracket expression (see below), '.' (matching any  single
43       character),  '^' (matching the null string at the beginning of a line),
44       '$' (matching the null string at the end of a line), a '\' followed  by
45       one  of the characters "^.[$()|*+?{\" (matching that character taken as
46       an ordinary character),  a  '\'  followed  by  any  other  character(!)
47       (matching  that character taken as an ordinary character, as if the '\'
48       had not been present(!)), or a single character with no other  signifi‐
49       cance  (matching  that character).  A '{' followed by a character other
50       than a digit is an ordinary character, not the beginning of a bound(!).
51       It is illegal to end an RE with '\'.
52
53       A bracket expression is a list of characters enclosed in "[]".  It nor‐
54       mally matches any single character from the list (but see  below).   If
55       the  list  begins  with  '^',  it matches any single character (but see
56       below) not from the rest of the list.  If two characters  in  the  list
57       are  separated  by '-', this is shorthand for the full range of charac‐
58       ters between those two (inclusive) in the collating sequence, for exam‐
59       ple,  "[0-9]" in ASCII matches any decimal digit.  It is illegal(!) for
60       two ranges to share an endpoint, for example, "a-c-e".  Ranges are very
61       collating-sequence-dependent,  and portable programs should avoid rely‐
62       ing on them.
63
64       To include a literal ']' in the list, make it the first character (fol‐
65       lowing a possible '^').  To include a literal '-', make it the first or
66       last character, or the second endpoint of a range.  To  use  a  literal
67       '-'  as  the first endpoint of a range, enclose it in "[." and ".]"  to
68       make it a collating element (see below).  With the exception  of  these
69       and  some  combinations using '[' (see next paragraphs), all other spe‐
70       cial characters, including '\', lose their special significance  within
71       a bracket expression.
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73       Within a bracket expression, a collating element (a character, a multi‐
74       character sequence that collates as if it were a single character, or a
75       collating-sequence  name  for  either) enclosed in "[." and ".]" stands
76       for the sequence of characters of that collating element.  The sequence
77       is  a  single  element  of  the  bracket  expression's list.  A bracket
78       expression containing a multicharacter collating element can thus match
79       more  than  one  character,  for  example,  if  the  collating sequence
80       includes a "ch" collating element, then the RE "[[.ch.]]*c" matches the
81       first five characters of "chchcc".
82
83       Within  a  bracket expression, a collating element enclosed in "[=" and
84       "=]" is an equivalence class, standing for the sequences of  characters
85       of  all  collating  elements  equivalent to that one, including itself.
86       (If there are no other equivalent collating elements, the treatment  is
87       as  if the enclosing delimiters were "[." and ".]".)  For example, if o
88       and o^  are  the  members  of  an  equivalence  class,  then  "[[=o=]]",
89       "[[=o^=]]",  and  "[oo^]"  are  all synonymous.  An equivalence class may
90       not(!) be an endpoint of a range.
91
92       Within a bracket expression, the name of a character class enclosed  in
93       "[:"  and  ":]" stands for the list of all characters belonging to that
94       class.  Standard character class names are:
95
96              alnum   digit   punct
97              alpha   graph   space
98              blank   lower   upper
99              cntrl   print   xdigit
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101       These stand for the character classes defined in wctype(3).   A  locale
102       may  provide  others.  A character class may not be used as an endpoint
103       of a range.
104
105       In the event that an RE could match more than one substring of a  given
106       string, the RE matches the one starting earliest in the string.  If the
107       RE could match more than one  substring  starting  at  that  point,  it
108       matches  the  longest.   Subexpressions also match the longest possible
109       substrings, subject to the constraint that the whole match be  as  long
110       as possible, with subexpressions starting earlier in the RE taking pri‐
111       ority over ones starting later.  Note that higher-level  subexpressions
112       thus take priority over their lower-level component subexpressions.
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114       Match  lengths  are  measured in characters, not collating elements.  A
115       null string is considered longer than no match at  all.   For  example,
116       "bb*"    matches    the    three    middle   characters   of   "abbbc",
117       "(wee|week)(knights|nights)"  matches  all  ten  characters  of  "week‐
118       nights",  when "(.*).*" is matched against "abc" the parenthesized sub‐
119       expression matches all three characters, and when  "(a*)*"  is  matched
120       against  "bc"  both  the  whole  RE and the parenthesized subexpression
121       match the null string.
122
123       If case-independent matching is specified, the effect is much as if all
124       case  distinctions  had vanished from the alphabet.  When an alphabetic
125       that exists in multiple cases appears as an ordinary character  outside
126       a  bracket  expression,  it  is  effectively transformed into a bracket
127       expression containing both cases,  for  example,  'x'  becomes  "[xX]".
128       When  it  appears inside a bracket expression, all case counterparts of
129       it are added to the bracket expression, so  that,  for  example,  "[x]"
130       becomes "[xX]" and "[^x]" becomes "[^xX]".
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132       No  particular  limit  is  imposed  on  the length of REs(!).  Programs
133       intended to be portable should not employ REs longer than 256 bytes, as
134       an  implementation  can refuse to accept such REs and remain POSIX-com‐
135       pliant.
136
137       Obsolete ("basic") regular  expressions  differ  in  several  respects.
138       '|',  '+',  and  '?' are ordinary characters and there is no equivalent
139       for their functionality.  The delimiters for bounds are "\{" and  "\}",
140       with  '{'  and  '}' by themselves ordinary characters.  The parentheses
141       for nested subexpressions are "\(" and "\)", with '(' and ')' by  them‐
142       selves ordinary characters.  '^' is an ordinary character except at the
143       beginning of the RE or(!) the beginning of a  parenthesized  subexpres‐
144       sion,  '$'  is  an ordinary character except at the end of the RE or(!)
145       the end of a parenthesized subexpression, and '*' is an ordinary  char‐
146       acter  if  it  appears at the beginning of the RE or the beginning of a
147       parenthesized subexpression (after a possible leading '^').
148
149       Finally, there is one new type of atom, a back reference: '\'  followed
150       by  a  nonzero  decimal digit d matches the same sequence of characters
151       matched by the dth parenthesized  subexpression  (numbering  subexpres‐
152       sions by the positions of their opening parentheses, left to right), so
153       that, for example, "\([bc]\)\1" matches "bb" or "cc" but not "bc".
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BUGS

156       Having two kinds of REs is a botch.
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158       The current POSIX.2 spec says that ')' is an ordinary character in  the
159       absence  of  an  unmatched  '(';  this was an unintentional result of a
160       wording error, and change is likely.  Avoid relying on it.
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162       Back references are a dreadful botch, posing major problems  for  effi‐
163       cient  implementations.   They  are also somewhat vaguely defined (does
164       "a\(\(b\)*\2\)*d" match "abbbd"?).  Avoid using them.
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166       POSIX.2's specification of case-independent  matching  is  vague.   The
167       "one  case implies all cases" definition given above is current consen‐
168       sus among implementors as to the right interpretation.
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AUTHOR

171       This page was taken from Henry Spencer's regex package.
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SEE ALSO

174       grep(1), regex(3)
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176       POSIX.2, section 2.8 (Regular Expression Notation).
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COLOPHON

179       This page is part of release 4.16 of the Linux  man-pages  project.   A
180       description  of  the project, information about reporting bugs, and the
181       latest    version    of    this    page,    can     be     found     at
182       https://www.kernel.org/doc/man-pages/.
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186                                  2009-01-12                          REGEX(7)
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