34.3.1 Syntax of Regular Expressions

Regular expressions have a syntax in which a few characters are special constructs and the rest are ordinary. An ordinary character is a simple regular expression that matches that character and nothing else. The special characters are ‘.’, ‘*’, ‘+’, ‘?’, ‘[’, ‘^’, ‘$’, and ‘\’; no new special characters will be defined in the future. The character ‘]’ is special if it ends a character alternative (see later). The character ‘-’ is special inside a character alternative. A ‘[:’ and balancing ‘:]’ enclose a character class inside a character alternative. Any other character appearing in a regular expression is ordinary, unless a ‘\’ precedes it.

For example, ‘f’ is not a special character, so it is ordinary, and therefore ‘f’ is a regular expression that matches the string ‘f’ and no other string. (It does not match the string ‘fg’, but it does match a part of that string.) Likewise, ‘o’ is a regular expression that matches only ‘o’.

Any two regular expressions a and b can be concatenated. The result is a regular expression that matches a string if a matches some amount of the beginning of that string and b matches the rest of the string.

As a simple example, we can concatenate the regular expressions ‘f’ and ‘o’ to get the regular expression ‘fo’, which matches only the string ‘fo’. Still trivial. To do something more powerful, you need to use one of the special regular expression constructs.

• Regexp Special		Special characters in regular expressions.
• Char Classes		Character classes used in regular expressions.
• Regexp Backslash		Backslash-sequences in regular expressions.

34.3.1.1 Special Characters in Regular Expressions

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 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 a very 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. In general, avoid expressions that can match the same string in multiple ways.

‘+’

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. However, the ending character of one range should not be the starting point of another one; for example, ‘[a-m-z]’ should be avoided.

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.

The usual regexp special characters are not special inside a character alternative. A completely different set of characters is special: ‘]’, ‘-’ and ‘^’. To include ‘]’ in a character alternative, put it at the beginning. To include ‘^’, put it anywhere but at the beginning. To include ‘-’, put it at the end. Thus, ‘[]^-]’ matches all three of these special characters. You cannot use ‘\’ to escape these three characters, since ‘\’ is not special here.

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:

1. If case-fold-search is non-nil, ‘[a-z]’ also matches upper-case letters.
2. A range is not affected by the locale’s collation sequence: it always represents the set of characters with codepoints ranging between those of its bounds, so that ‘[a-z]’ matches only ASCII letters, even outside the C or POSIX locale.
3. If the lower bound of a range is greater than its upper bound, the range is empty and represents no characters. Thus, ‘[z-a]’ always fails to match, and ‘[^z-a]’ matches any character, including newline. However, a reversed range should always be from the letter ‘z’ to the letter ‘a’ to make it clear that it is not a typo; for example, ‘[+-*/]’ should be avoided, because it matches only ‘/’ rather than the likely-intended four characters.

Some kinds of character alternatives are not the best style even though they have a well-defined meaning in Emacs. They include:

1. Although a range’s bound can be almost any character, it is better style to stay within natural sequences of ASCII letters and digits because most people have not memorized character code tables. For example, ‘[.-9]’ is less clear than ‘[./0-9]’, and ‘[`-~]’ is less clear than ‘[`a-z{|}~]’. Unicode character escapes can help here; for example, for most programmers ‘[ก-ฺ฿-๛]’ is less clear than ‘[\u0E01-\u0E3A\u0E3F-\u0E5B]’.
2. Although a character alternative can include duplicates, it is better style to avoid them. For example, ‘[XYa-yYb-zX]’ is less clear than ‘[XYa-z]’.
3. Although a range can denote just one, two, or three characters, it is simpler to list the characters. For example, ‘[a-a0]’ is less clear than ‘[a0]’, ‘[i-j]’ is less clear than ‘[ij]’, and ‘[i-k]’ is less clear than ‘[ijk]’.
4. Although a ‘-’ can appear at the beginning of a character alternative or as the upper bound of a range, it is better style to put ‘-’ by itself at the end of a character alternative. For example, although ‘[-a-z]’ is valid, ‘[a-z-]’ is better style; and although ‘[*--]’ is valid, ‘[*+,-]’ is clearer.

‘[^ … ]’

‘[^’ begins a complemented character alternative. This matches any character except the ones specified. Thus, ‘[^a-z0-9A-Z]’ matches all characters except ASCII 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. See 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 (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 ‘\’ 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.

34.3.1.2 Character Classes

Below is a table of the classes you can use in a character alternative, and what they mean. Note that the ‘[’ and ‘]’ characters that enclose the class name are part of the name, so a regular expression using these classes needs one more pair of brackets. For example, a regular expression matching a sequence of one or more letters and digits would be ‘[[:alnum:]]+’, not ‘[:alnum:]+’.

‘[:ascii:]’

This matches any ASCII character (codes 0–127).

‘[:alnum:]’

This matches any letter or digit. For multibyte characters, it matches characters whose Unicode ‘general-category’ property (see Character Properties) indicates they are alphabetic or decimal number characters.

‘[:alpha:]’

This matches any letter. For multibyte characters, it matches characters whose Unicode ‘general-category’ property (see Character Properties) indicates they are alphabetic characters.

‘[:blank:]’

This matches horizontal whitespace, as defined by Annex C of the Unicode Technical Standard #18. In particular, it matches spaces, tabs, and other characters whose Unicode ‘general-category’ property (see Character Properties) indicates they are spacing separators.

‘[:cntrl:]’

This matches any character whose code is in the range 0–31.

‘[:digit:]’

This matches ‘0’ through ‘9’. Thus, ‘[-+[:digit:]]’ matches any digit, as well as ‘+’ and ‘-’.

‘[:graph:]’

This matches graphic characters—everything except whitespace, ASCII and non-ASCII control characters, surrogates, and codepoints unassigned by Unicode, as indicated by the Unicode ‘general-category’ property (see Character Properties).

‘[:lower:]’

This matches any lower-case letter, as determined by the current case table (see Case Tables). If case-fold-search is non-nil, this also matches any upper-case letter.

‘[:multibyte:]’

This matches any multibyte character (see Text Representations).

‘[:nonascii:]’

This matches any non-ASCII character.

‘[:print:]’

This matches any printing character—either whitespace, or a graphic character matched by ‘[:graph:]’.

‘[:punct:]’

This matches any punctuation character. (At present, for multibyte characters, it matches anything that has non-word syntax.)

‘[:space:]’

This matches any character that has whitespace syntax (see Syntax Class Table).

‘[:unibyte:]’

This matches any unibyte character (see Text Representations).

‘[:upper:]’

This matches any upper-case letter, as determined by the current case table (see Case Tables). If case-fold-search is non-nil, this also matches any lower-case letter.

‘[:word:]’

This matches any character that has word syntax (see Syntax Class Table).

‘[:xdigit:]’

This matches the hexadecimal digits: ‘0’ through ‘9’, ‘a’ through ‘f’ and ‘A’ through ‘F’.

34.3.1.3 Backslash Constructs in Regular Expressions

For the most part, ‘\’ followed by any character matches only that character. However, there are several exceptions: certain sequences starting with ‘\’ that have special meanings. Here is a table of the special ‘\’ constructs.

‘\|’

specifies an alternative. Two regular expressions a and b with ‘\|’ in between form an expression that matches anything that either a or b matches.

Thus, ‘foo\|bar’ matches either ‘foo’ or ‘bar’ but no other string.

‘\|’ applies to the largest possible surrounding expressions. Only a surrounding ‘\( … \)’ grouping can limit the grouping power of ‘\|’.

If you need full backtracking capability to handle multiple uses of ‘\|’, use the POSIX regular expression functions (see POSIX Regexps).

‘\{m\}’

is a postfix operator that repeats the previous pattern exactly m times. Thus, ‘x\{5\}’ matches the string ‘xxxxx’ and nothing else. ‘c[ad]\{3\}r’ matches string such as ‘caaar’, ‘cdddr’, ‘cadar’, and so on.

‘\{m,n\}’

is a more general postfix operator that specifies repetition with a minimum of m repeats and a maximum of n repeats. If m is omitted, the minimum is 0; if n is omitted, there is no maximum. For both forms, m and n, if specified, may be no larger than 2**16 - 1 .

For example, ‘c[ad]\{1,2\}r’ matches the strings ‘car’, ‘cdr’, ‘caar’, ‘cadr’, ‘cdar’, and ‘cddr’, and nothing else.\ ‘\{0,1\}’ or ‘\{,1\}’ is equivalent to ‘?’.\ ‘\{0,\}’ or ‘\{,\}’ is equivalent to ‘*’.\ ‘\{1,\}’ is equivalent to ‘+’.

‘\( … \)’

is a grouping construct that serves three purposes:

1. To enclose a set of ‘\|’ alternatives for other operations. Thus, the regular expression ‘\(foo\|bar\)x’ matches either ‘foox’ or ‘barx’.
2. To enclose a complicated expression for the postfix operators ‘*’, ‘+’ and ‘?’ to operate on. Thus, ‘ba\(na\)*’ matches ‘ba’, ‘bana’, ‘banana’, ‘bananana’, etc., with any number (zero or more) of ‘na’ strings.
3. To record a matched substring for future reference with ‘\digit’ (see below).

This last application is not a consequence of the idea of a parenthetical grouping; it is a separate feature that was assigned as a second meaning to the same ‘\( … \)’ construct because, in practice, there was usually no conflict between the two meanings. But occasionally there is a conflict, and that led to the introduction of shy groups.

‘\(?: … \)’

is the shy group construct. A shy group serves the first two purposes of an ordinary group (controlling the nesting of other operators), but it does not get a number, so you cannot refer back to its value with ‘\digit’. Shy groups are particularly useful for mechanically-constructed regular expressions, because they can be added automatically without altering the numbering of ordinary, non-shy groups.

Shy groups are also called non-capturing or unnumbered groups.

‘\(?num: … \)’

is the explicitly numbered group construct. Normal groups get their number implicitly, based on their position, which can be inconvenient. This construct allows you to force a particular group number. There is no particular restriction on the numbering, e.g., you can have several groups with the same number in which case the last one to match (i.e., the rightmost match) will win. Implicitly numbered groups always get the smallest integer larger than the one of any previous group.

‘\digit’

matches the same text that matched the digitth occurrence of a grouping (‘\( … \)’) construct.

In other words, after the end of a group, the matcher remembers the beginning and end of the text matched by that group. Later on in the regular expression you can use ‘\’ followed by digit to match that same text, whatever it may have been.

The strings matching the first nine grouping constructs appearing in the entire regular expression passed to a search or matching function are assigned numbers 1 through 9 in the order that the open parentheses appear in the regular expression. So you can use ‘\1’ through ‘\9’ to refer to the text matched by the corresponding grouping constructs.

For example, ‘\(.*\)\1’ matches any newline-free string that is composed of two identical halves. The ‘\(.*\)’ matches the first half, which may be anything, but the ‘\1’ that follows must match the same exact text.

If a ‘\( … \)’ construct matches more than once (which can happen, for instance, if it is followed by ‘*’), only the last match is recorded.

If a particular grouping construct in the regular expression was never matched—for instance, if it appears inside of an alternative that wasn’t used, or inside of a repetition that repeated zero times—then the corresponding ‘\digit’ construct never matches anything. To use an artificial example, ‘\(foo\(b*\)\|lose\)\2’ cannot match ‘lose’: the second alternative inside the larger group matches it, but then ‘\2’ is undefined and can’t match anything. But it can match ‘foobb’, because the first alternative matches ‘foob’ and ‘\2’ matches ‘b’.

‘\w’

matches any word-constituent character. The editor syntax table determines which characters these are. See Syntax Tables.

‘\W’

matches any character that is not a word constituent.

‘\scode’

matches any character whose syntax is code. Here code is a character that represents a syntax code: thus, ‘w’ for word constituent, ‘-’ for whitespace, ‘(’ for open parenthesis, etc. To represent whitespace syntax, use either ‘-’ or a space character. See Syntax Class Table, for a list of syntax codes and the characters that stand for them.

‘\Scode’

matches any character whose syntax is not code.

‘\cc’

matches any character whose category is c. Here c is a character that represents a category: thus, ‘c’ for Chinese characters or ‘g’ for Greek characters in the standard category table. You can see the list of all the currently defined categories with M-x describe-categories RET. You can also define your own categories in addition to the standard ones using the define-category function (see Categories).

‘\Cc’

matches any character whose category is not c.

The following regular expression constructs match the empty string—that is, they don’t use up any characters—but whether they match depends on the context. For all, the beginning and end of the accessible portion of the buffer are treated as if they were the actual beginning and end of the buffer.

‘\`’

matches the empty string, but only at the beginning of the buffer or string being matched against.

‘\'’

matches the empty string, but only at the end of the buffer or string being matched against.

‘\=’

matches the empty string, but only at point. (This construct is not defined when matching against a string.)

‘\b’

matches the empty string, but only at the beginning or end of a word. Thus, ‘\bfoo\b’ matches any occurrence of ‘foo’ as a separate word. ‘\bballs?\b’ matches ‘ball’ or ‘balls’ as a separate word.

‘\b’ matches at the beginning or end of the buffer (or string) regardless of what text appears next to it.

‘\B’

matches the empty string, but not at the beginning or end of a word, nor at the beginning or end of the buffer (or string).

‘\<’

matches the empty string, but only at the beginning of a word. ‘\<’ matches at the beginning of the buffer (or string) only if a word-constituent character follows.

‘\>’

matches the empty string, but only at the end of a word. ‘\>’ matches at the end of the buffer (or string) only if the contents end with a word-constituent character.

‘\_<’

matches the empty string, but only at the beginning of a symbol. A symbol is a sequence of one or more word or symbol constituent characters. ‘\_<’ matches at the beginning of the buffer (or string) only if a symbol-constituent character follows.

‘\_>’

matches the empty string, but only at the end of a symbol. ‘\_>’ matches at the end of the buffer (or string) only if the contents end with a symbol-constituent character.

Not every string is a valid regular expression. For example, a string that ends inside a character alternative without a terminating ‘]’ is invalid, and so is a string that ends with a single ‘\’. If an invalid regular expression is passed to any of the search functions, an invalid-regexp error is signaled.