Here is a list of the characters that are special in a regular expression.
‘*’ 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 this 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.
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’.)
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. However, the ending character of one range should not be the starting point of another one; for example, ‘[a-m-z]’ should be avoided.
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 as the upper bound of 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.
The following aspects of ranges are specific to Emacs, in that POSIX allows but does not require this behavior and programs other than Emacs may behave differently:
nil, ‘[a-z]’ also matches upper-case letters.
A character alternative can also specify named character classes
(see 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.
A character class should not appear as the lower or upper bound
of a range.
‘^’ 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
You can specify named character classes, just like in character
alternatives. For instance, ‘[^[:ascii:]]’ matches any
non-ASCII character. See Char Classes.
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 ‘\(’, ‘\(?:’
When matching a string instead of a buffer, ‘$’ matches at the end of the string or before a newline character.
For historical compatibility reasons, ‘$’ can be used only at the
end of the regular expression, or before ‘\)’ or ‘\|’.
Because ‘\’ quotes special characters, ‘\$’ is a regular expression that matches only ‘$’, and ‘\[’ is a regular expression that matches only ‘[’, and so on.
Note that ‘\’ also has special meaning in the read syntax of Lisp
strings (see String Type), and must be quoted with ‘\’. For
example, the regular expression that matches the ‘\’ character is
‘\\’. To write a Lisp string that contains the characters
‘\\’, Lisp syntax requires you to quote each ‘\’ with another
‘\’. Therefore, the read syntax for a regular expression matching
Please note: For historical compatibility, special characters are treated as ordinary ones if they are in contexts where their special meanings make no sense. For example, ‘*foo’ treats ‘*’ as ordinary since there is no preceding expression on which the ‘*’ can act. It is poor practice to depend on this behavior; quote the special character anyway, regardless of where it appears.
As a ‘\’ is not special inside a character alternative, it can
never remove the special meaning of ‘-’ or ‘]’. So you
should not quote these characters when they have no special meaning
either. This would not clarify anything, since backslashes can
legitimately precede these characters where they have special
meaning, as in ‘[^\]’ (
"[^\\]" 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.