(This documentation is derived from Tom Lords documentation for the Free Software Foundation regular expression library)
Copyright (C) 1996 Tom Lord Berkeley, CA USA
Permission to use, copy, modify, distribute, and sell this documentation for any purpose is hereby granted without fee, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation.
The Posix Basic Regular Expression language is a notation for describing textual patterns. Regexps are typically used by comparing them to a string to see if that string matches the pattern, or by searching within a string for a substring that matches.
This chapter introduces the Posix regexp notation. This is not a formal or precise definition of Posix regexps -- it is an intuitive and hopefully expository description of them.
In the simplest cases, a regexp is just a literal string that must match exactly. For example, the pattern:
regexp
matches the string "regexp" and no others.
Some characters have a special meaning when they occur in a regexp.
They aren't matched literally as in the previous example, but instead
denote a more general pattern. For example, the character *
is used to indicate that the preceeding element of a regexp may be
repeated 0, 1, or more times. In the pattern:
smooo*th
the * indicates that the preceeding o can be repeated 0 or
more times. So the pattern matches:
smooth smoooth smooooth smoooooth ...
Suppose you want to write a pattern that literally matches a special
character like * -- in other words, you don't want to * to
indicate a permissible repetition, but to match * literally. This
is accomplished by quoting the special character with a backslash.
The pattern:
smoo\*th
matches the string:
smoo*th
and no other strings.
In seven cases, the pattern is reversed -- a backslash makes the
character special instead of making a special character normal. The
characters +, ?, |, (, and ) are
normal but the sequences \+, \?, \|, \(,
\), \{, and \} are special (their meaning is
described later).
The remaining sections of this chapter introduce and explain the various special characters that can occur in regexps.
A literal regexp is a string which contains no special characters. A literal regexp matches an identical string, but no other characters. For example:
literally
matches
literally
and nothing else.
Generally, whitespace characters, numbers, and letters are not special. Some punctuation characters are special and some are not (the syntax summary at the end of this chapter makes a convenient reference for which characters are special and which aren't).
This section introduces the special characters . and [.
. matches any character except the NULL character. For example:
p.ck
matches
pick pack puck pbck pcck p.ck ...
[ begins a character set. A character set is similar to
. in that it matches not a single, literal character, but any
of a set of characters. [ is different from . in that
with [, you define the set of characters explicitly.
There are three basic forms a character set can take.
In the first form, the character set is spelled out:
[<cset-spec>] -- every character in <cset-spec> is in the set.
In the second form, the character set indicated is the negation of a character set is explicitly spelled out:
[^<cset-spec>] -- every character *not* in <cset-spec> is in the set.
A <cset-spec> is more or less an explicit enumeration of a set
of characters. It can be written as a string of individual characters:
[aeiou]
or as a range of characters:
[0-9]
These two forms can be mixed:
[A-za-z0-9_$]
Note that special regexp characters (such as *) are not
special within a character set. -, as illustrated above,
is special, except, as illustrated below, when it is the first
character mentioned.
This is a four-character set:
[-+*/]
The third form of a character set makes use of a pre-defined "character class":
[[:class-name:]] -- every character described by class-name is in the set.
The supported character classes are:
alnum - the set of alpha-numeric characters alpha - the set of alphabetic characters blank - tab and space cntrl - the control characters digit - decimal digits graph - all printable characters except space lower - lower case letters print - the "printable" characters punct - punctuation space - whitespace characters upper - upper case letters xdigit - hexidecimal digits
Finally, character class sets can also be inverted:
[^[:space:]] - all non-whitespace characters
Character sets can be used in a regular expression anywhere a literal character can.
A subexpression is a regular expression enclosed in \( and
\). A subexpression can be used anywhere a single character or
character set can be used.
Subexpressions are useful for grouping regexp constructs. For example,
the repeat operator, *, usually applies to just the preceeding
character. Recall that:
smooo*th
matches
smooth smoooth ...
Using a subexpression, we can apply * to a longer string:
banan\(an\)*a
matches
banana bananana banananana ...
Subexpressions also have a special meaning with regard to backreferences and substitutions (see See Backreferences, Extractions and Substitutions).
* is the repeat operator. It applies to the preceeding
character, character set, subexpression or backreference. It indicates
that the preceeding element can be matched 0 or more times:
bana\(na\)*
matches
bana banana bananana banananana ...
\+ is similar to * except that \+ requires the
preceeding element to be matched at least once. So while:
bana\(na\)*
matches
bana
bana(na\)\+
does not. Both match
banana bananana banananana ...
Thus, bana\(na\)+ is short-hand for banana\(na\)*.
\? indicates that the preceeding character, character set, or
subexpression is optional. It is permitted to match, or to be skipped:
CSNY\?
matches both
CSN
and
CSNY
An interval expression, \{m,n\} where m and n are
non-negative integers with n >= m, applies to the preceeding
character, character set, subexpression or backreference. It indicates
that the preceeding element must match at least m times and may
match as many as n times.
For example:
c\([ad]\)\{1,4\}
matches
car cdr caar cdar ... caaar cdaar ... cadddr cddddr
An alternative is written:
regexp-1\|regexp-2\|regexp-3\|...
It matches anything matched by some regexp-n. For example:
Crosby, Stills, \(and Nash\|Nash, and Young\)
matches
Crosby, Stills, and Nash
and
Crosby, Stills, Nash, and Young
A backreference is written \n where n is some single digit
other than 0. To be a valid backreference, there must be at least
n parenthesized subexpressions in the pattern prior to the
backreference.
A backreference matches a literal copy of whatever was matched by the corresponding subexpression. For example,
\(.*\)-\1
matches:
go-go ha-ha wakka-wakka ...
In some applications, subexpressions are used to extract substrings.
For example, Emacs has the functions match-beginnning and
match-end which report the positions of strings matched by
subexpressions. These functions use the same numbering scheme for
subexpressions as backreferences, with the additional rule that
subexpression 0 is defined to be the whole regexp.
In some applications, subexpressions are used in string substitution. This again uses the backreference numbering scheme. For example, this sed command:
s/From:.*<\(.*\)>/To: \1/
first matches the line:
From: Joe Schmoe <schmoe@uspringfield.edu>
when it does, subexpression 1 matches "schmoe@uspringfield.edu". The command replaces the matched line with "To: \1" after doing subexpression substitution on it to get:
To: schmoe@uspringfield.edu
In summary, regexps can be:
abcd -- matching a string literally
. -- matching everything except NULL
[a-z_?], ^[a-z_?], [[:alpha:]] and
[^[:alpha:]] -- matching character sets
\(subexp\) -- grouping an expression into a subexpression.
\n -- match a copy of whatever was matched by the nth subexpression.
The following special characters and sequences can be applied to a character, character set, subexpression, or backreference:
* -- repeat the preceeding element 0 or more times.
\+ -- repeat the preceeding element 1 or more times.
\? -- match the preceeding element 0 or 1 time.
{m,n} -- match the preceeding element at least m, and as
many as n times.
regexp-1\|regexp-2\|.. -- match any regexp-n.
A special character, like . or * can be made into a literal
character by prefixing it with \.
A special sequence, like \+ or \? can be made into a
literal character by dropping the \.
Sometimes a regular expression appears to be ambiguous. For example, suppose we compare the pattern:
begin\|beginning
to the string
beginning
either just the first 5 characters will match, or the whole string will match.
In every case like this, the longer match is preferred. The whole string will match.
Sometimes there is ambiguity not about how many characters to match, but
where the subexpressions occur within the match. This can effect
extraction functions like Emacs' match-beginning or rewrite
functions like sed's s command. For example, consider matching
the pattern:
b\(\[^q]*\)\(ing\)?
against the string
beginning
One possibility is that the first subexpression matches "eginning" and the second is skipped. Another possibility is that the first subexpression matches "eginn" and the second matches "ing".
The rule is that consistant with matching as many characters as possible, the length of lower numbered subexpressions is maximized in preference to maximizing the length of later subexpressions.
In the case of the above example, the two possible matches are equal in overall length. Therefore, it comes down to maximizing the lower-numbered subexpression, \1. The correct answer is that \1 matches "eginning" and \2 is skipped.
Sometimes a subexpression matches a substring of no characters. This
happens when `f\(o*\)' matches the string `fum'. (It really
matches just the `f'.) In this case, both of the offsets identify
the point in the string where the null substring was found. In this
example, the offsets are both 1.
Sometimes the entire regular expression can match without using some of
its subexpressions at all--for example, when `ba\(na\)*' matches the
string `ba', the parenthetical subexpression is not used. When
this happens, regexec stores -1 in both fields of the
element for that subexpression.
Sometimes matching the entire regular expression can match a particular
subexpression more than once--for example, when `ba\(na\)*'
matches the string `bananana', the parenthetical subexpression
matches three times. When this happens, regexec usually stores
the offsets of the last part of the string that matched the
subexpression. In the case of `bananana', these offsets are
6 and 8.
But the last match is not always the one that is chosen. It's more
accurate to say that the last opportunity to match is the one
that takes precedence. What this means is that when one subexpression
appears within another, then the results reported for the inner
subexpression reflect whatever happened on the last match of the outer
subexpression. For an example, consider `\(ba\(na\)*s \)*' matching
the string `bananas bas '. The last time the inner expression
actually matches is near the end of the first word. But it is
considered again in the second word, and fails to match there.
regexec reports nonuse of the "na" subexpression.
Another place where this rule applies is when the regular expression
`\(ba\(na\)*s \|nefer\(ti\)* \)*' matches `bananas nefertiti'.
The "na" subexpression does match in the first word, but it doesn't
match in the second word because the other alternative is used there.
Once again, the second repetition of the outer subexpression overrides
the first, and within that second repetition, the "na" subexpression
is not used. So regexec reports nonuse of the "na"
subexpression.