elisp: Regexp Special
33.3.1.1 Special Characters in Regular Expressions
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Here is a list of the characters that are special in a regular
expression.
‘.’ (Period)
is a special character that matches any single character except a
newline. Using concatenation, we can make regular expressions like
‘a.b’, which matches any three-character string that begins with
‘a’ and ends with ‘b’.
‘*’
is not a construct by itself; it is a postfix operator that means
to match the preceding regular expression repetitively as many
times as possible. Thus, ‘o*’ matches any number of ‘o’s
(including no ‘o’s).
‘*’ always applies to the _smallest_ possible preceding expression.
Thus, ‘fo*’ has a repeating ‘o’, not a repeating ‘fo’. It matches
‘f’, ‘fo’, ‘foo’, and so on.
The matcher processes a ‘*’ construct by matching, immediately, as
many repetitions as can be found. Then it continues with the rest
of the pattern. If that fails, backtracking occurs, discarding
some of the matches of the ‘*’-modified construct in the hope that
that will make it possible to match the rest of the pattern. For
example, in matching ‘ca*ar’ against the string ‘caaar’, the ‘a*’
first tries to match all three ‘a’s; but the rest of the pattern is
‘ar’ and there is only ‘r’ left to match, so this try fails. The
next alternative is for ‘a*’ to match only two ‘a’s. With this
choice, the rest of the regexp matches successfully.
*Warning:* Nested repetition operators can run for an indefinitely
long time, if they lead to ambiguous matching. For example, trying
to match the regular expression ‘\(x+y*\)*a’ against the string
‘xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz’ could take hours before it
ultimately fails. Emacs must try each way of grouping the ‘x’s
before concluding that none of them can work. Even worse,
‘\(x*\)*’ can match the null string in infinitely many ways, so it
causes an infinite loop. To avoid these problems, check nested
repetitions carefully, to make sure that they do not cause
combinatorial explosions in backtracking.
‘+’
is a postfix operator, similar to ‘*’ except that it must match the
preceding expression at least once. So, for example, ‘ca+r’
matches the strings ‘car’ and ‘caaaar’ but not the string ‘cr’,
whereas ‘ca*r’ matches all three strings.
‘?’
is a postfix operator, similar to ‘*’ except that it must match the
preceding expression either once or not at all. For example,
‘ca?r’ matches ‘car’ or ‘cr’; nothing else.
‘*?’, ‘+?’, ‘??’
These are “non-greedy” variants of the operators ‘*’, ‘+’ and ‘?’.
Where those operators match the largest possible substring
(consistent with matching the entire containing expression), the
non-greedy variants match the smallest possible substring
(consistent with matching the entire containing expression).
For example, the regular expression ‘c[ad]*a’ when applied to the
string ‘cdaaada’ matches the whole string; but the regular
expression ‘c[ad]*?a’, applied to that same string, matches just
‘cda’. (The smallest possible match here for ‘[ad]*?’ that permits
the whole expression to match is ‘d’.)
‘[ ... ]’
is a “character alternative”, which begins with ‘[’ and is
terminated by ‘]’. In the simplest case, the characters between
the two brackets are what this character alternative can match.
Thus, ‘[ad]’ matches either one ‘a’ or one ‘d’, and ‘[ad]*’ matches
any string composed of just ‘a’s and ‘d’s (including the empty
string). It follows that ‘c[ad]*r’ matches ‘cr’, ‘car’, ‘cdr’,
‘caddaar’, etc.
You can also include character ranges in a character alternative,
by writing the starting and ending characters with a ‘-’ between
them. Thus, ‘[a-z]’ matches any lower-case ASCII letter. Ranges
may be intermixed freely with individual characters, as in
‘[a-z$%.]’, which matches any lower case ASCII letter or ‘$’, ‘%’
or period.
If ‘case-fold-search’ is non-‘nil’, ‘[a-z]’ also matches upper-case
letters. Note that a range like ‘[a-z]’ is not affected by the
locale’s collation sequence, it always represents a sequence in
ASCII order.
Note also that the usual regexp special characters are not special
inside a character alternative. A completely different set of
characters is special inside character alternatives: ‘]’, ‘-’ and
‘^’.
To include a ‘]’ in a character alternative, you must make it the
first character. For example, ‘[]a]’ matches ‘]’ or ‘a’. To
include a ‘-’, write ‘-’ as the first or last character of the
character alternative, or put it after a range. Thus, ‘[]-]’
matches both ‘]’ and ‘-’. (As explained below, you cannot use ‘\]’
to include a ‘]’ inside a character alternative, since ‘\’ is not
special there.)
To include ‘^’ in a character alternative, put it anywhere but at
the beginning.
If a range starts with a unibyte character C and ends with a
multibyte character C2, the range is divided into two parts: one
spans the unibyte characters ‘C..?\377’, the other the multibyte
characters ‘C1..C2’, where C1 is the first character of the charset
to which C2 belongs.
A character alternative can also specify named character classes
(
Char Classes). This is a POSIX feature. For example,
‘[[:ascii:]]’ matches any ASCII character. Using a character class
is equivalent to mentioning each of the characters in that class;
but the latter is not feasible in practice, since some classes
include thousands of different characters.
‘[^ ... ]’
‘[^’ begins a “complemented character alternative”. This matches
any character except the ones specified. Thus, ‘[^a-z0-9A-Z]’
matches all characters _except_ letters and digits.
‘^’ is not special in a character alternative unless it is the
first character. The character following the ‘^’ is treated as if
it were first (in other words, ‘-’ and ‘]’ are not special there).
A complemented character alternative can match a newline, unless
newline is mentioned as one of the characters not to match. This
is in contrast to the handling of regexps in programs such as
‘grep’.
You can specify named character classes, just like in character
alternatives. For instance, ‘[^[:ascii:]]’ matches any non-ASCII
character. Char Classes.
‘^’
When matching a buffer, ‘^’ matches the empty string, but only at
the beginning of a line in the text being matched (or the beginning
of the accessible portion of the buffer). Otherwise it fails to
match anything. Thus, ‘^foo’ matches a ‘foo’ that occurs at the
beginning of a line.
When matching a string instead of a buffer, ‘^’ matches at the
beginning of the string or after a newline character.
For historical compatibility reasons, ‘^’ can be used only at the
beginning of the regular expression, or after ‘\(’, ‘\(?:’ or ‘\|’.
‘$’
is similar to ‘^’ but matches only at the end of a line (or the end
of the accessible portion of the buffer). Thus, ‘x+$’ matches a
string of one ‘x’ or more at the end of a line.
When matching a string instead of a buffer, ‘$’ matches at the end
of the string or before a newline character.
For historical compatibility reasons, ‘$’ can be used only at the
end of the regular expression, or before ‘\)’ or ‘\|’.
‘\’
has two functions: it quotes the special characters (including
‘\’), and it introduces additional special constructs.
Because ‘\’ quotes special characters, ‘\$’ is a regular expression
that matches only ‘$’, and ‘\[’ is a regular expression that
matches only ‘[’, and so on.
Note that ‘\’ also has special meaning in the read syntax of Lisp
strings (String Type), and must be quoted with ‘\’. For
example, the regular expression that matches the ‘\’ character is
‘\\’. To write a Lisp string that contains the characters ‘\\’,
Lisp syntax requires you to quote each ‘\’ with another ‘\’.
Therefore, the read syntax for a regular expression matching ‘\’ is
‘"\\\\"’.
*Please note:* For historical compatibility, special characters are
treated as ordinary ones if they are in contexts where their special
meanings make no sense. For example, ‘*foo’ treats ‘*’ as ordinary
since there is no preceding expression on which the ‘*’ can act. It is
poor practice to depend on this behavior; quote the special character
anyway, regardless of where it appears.
As a ‘\’ is not special inside a character alternative, it can never
remove the special meaning of ‘-’ or ‘]’. So you should not quote these
characters when they have no special meaning either. This would not
clarify anything, since backslashes can legitimately precede these
characters where they _have_ special meaning, as in ‘[^\]’ (‘"[^\\]"’
for Lisp string syntax), which matches any single character except a
backslash.
In practice, most ‘]’ that occur in regular expressions close a
character alternative and hence are special. However, occasionally a
regular expression may try to match a complex pattern of literal ‘[’ and
‘]’. In such situations, it sometimes may be necessary to carefully
parse the regexp from the start to determine which square brackets
enclose a character alternative. For example, ‘[^][]]’ consists of the
complemented character alternative ‘[^][]’ (which matches any single
character that is not a square bracket), followed by a literal ‘]’.
The exact rules are that at the beginning of a regexp, ‘[’ is special
and ‘]’ not. This lasts until the first unquoted ‘[’, after which we
are in a character alternative; ‘[’ is no longer special (except when it
starts a character class) but ‘]’ is special, unless it immediately
follows the special ‘[’ or that ‘[’ followed by a ‘^’. This lasts until
the next special ‘]’ that does not end a character class. This ends the
character alternative and restores the ordinary syntax of regular
expressions; an unquoted ‘[’ is special again and a ‘]’ not.
Info Catalog
elisp: Syntax of Regexps
elisp: Char Classes