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### 12.5 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. 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 ‘ff’.) 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 characters. Here is a list of them:

. (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 quantifying suffix operator that means to repeat the preceding regular expression as many times as possible. In ‘fo*’, the ‘*’ applies to the ‘o’, so ‘fo*’ matches one ‘f’ followed by any number of ‘o’s. The case of zero ‘o’s is allowed: ‘fo*’ does match ‘f’.

*’ always applies to the smallest possible preceding expression. Thus, ‘fo*’ has a repeating ‘o’, not a repeating ‘fo’.

The matcher processes a ‘*’ construct by matching, immediately, as many repetitions as can be found; it is "greedy". Then it continues with the rest of the pattern. If that fails, backtracking occurs, discarding some of the matches of the ‘*’-modified construct in case that makes 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.

Nested repetition operators can be extremely slow if they specify backtracking loops. For example, it could take hours for the regular expression ‘\(x+y*\)*a’ to match the sequence ‘xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxz’. The slowness is because Emacs must try each imaginable way of grouping the 35 ‘x’’s before concluding that none of them can work. To make sure your regular expressions run fast, check nested repetitions carefully.

+

is a quantifying suffix operator similar to ‘*’ except that the preceding expression must match at least once. It is also "greedy". 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 quantifying suffix operator similar to ‘*’, except that the preceding expression can match either once or not at all. For example, ‘ca?r’ matches ‘car’ or ‘cr’, but does not match anything else.

*?

works just like ‘*’, except that rather than matching the longest match, it matches the shortest match. ‘*?’ is known as a non-greedy quantifier, a regexp construct borrowed from Perl.

This construct is very useful for when you want to match the text inside a pair of delimiters. For instance, ‘/\*.*?\*/’ will match C comments in a string. This could not easily be achieved without the use of a non-greedy quantifier.

This construct has not been available prior to XEmacs 20.4. It is not available in FSF Emacs.

+?

is the non-greedy version of ‘+’.

??

is the non-greedy version of ‘?’.

\{n,m\}

serves as an interval quantifier, analogous to ‘*’ or ‘+’, but specifies that the expression must match at least n times, but no more than m times. This syntax is supported by most Unix regexp utilities, and has been introduced to XEmacs for the version 20.3.

Unfortunately, the non-greedy version of this quantifier does not exist currently, although it does in Perl.

[ … ]

[’ begins a character set, which is terminated by a ‘]’. In the simplest case, the characters between the two brackets form the set. 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), from which it follows that ‘c[ad]*r’ matches ‘cr’, ‘car’, ‘cdr’, ‘caddaar’, etc.

The usual regular expression special characters are not special inside a character set. A completely different set of special characters exists inside character sets: ‘]’, ‘-’ and ‘^’.

-’ is used for ranges of characters. To write a range, write two characters with a ‘-’ between them. Thus, ‘[a-z]’ matches any lower case letter. Ranges may be intermixed freely with individual characters, as in ‘[a-z\$%.]’, which matches any lower case letter or ‘\$’, ‘%’, or a period.

To include a ‘]’ in a character set, make it the first character. For example, ‘[]a]’ matches ‘]’ or ‘a’. To include a ‘-’, write ‘-’ as the first character in the set, or put it immediately after a range. (You can replace one individual character c with the range ‘c-c’ to make a place to put the ‘-’.) There is no way to write a set containing just ‘-’ and ‘]’.

To include ‘^’ in a set, put it anywhere but at the beginning of the set.

[^ … ]

[^’ begins a complement character set, which 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 set unless it is the first character. The character following the ‘^’ is treated as if it were first (thus, ‘-’ and ‘]’ are not special there).

Note that a complement character set can match a newline, unless newline is mentioned as one of the characters not to match.

^

is a special character that matches the empty string, but only at the beginning of a line in the text being matched. 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 ‘\n’.

\$

is similar to ‘^’ but matches only at the end of a line. 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 ‘\n’.

\

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.

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.

For the most part, ‘\’ followed by any character matches only that character. However, there are several exceptions: characters that, when preceded by ‘\’, are special constructs. Such characters are always ordinary when encountered on their own. Here is a table of ‘\’ 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 ‘\|’.

Full backtracking capability exists to handle multiple uses of ‘\|’.

\( … \)

is a grouping construct that serves three purposes:

1. To enclose a set of ‘\|’ alternatives for other operations. Thus, ‘\(foo\|bar\)x’ matches either ‘foox’ or ‘barx’.
2. To enclose an expression for a suffix operator such as ‘*’ to act on. Thus, ‘ba\(na\)*’ matches ‘bananana’, etc., with any (zero or more) number of ‘na’ strings.
3. To record a matched substring for future reference.

This last application is not a consequence of the idea of a parenthetical grouping; it is a separate feature that happens to be assigned as a second meaning to the same ‘\( … \)’ construct because there is no conflict in practice between the two meanings. Here is an explanation of this feature:

\digit

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

In other words, after the end of a ‘\( … \)’ construct. the matcher remembers the beginning and end of the text matched by that construct. Then, 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 ‘\( … \)’ constructs appearing in a regular expression 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 ‘\( … \)’ 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.

\(?: … \)

is called a shy grouping operator, and it is used just like ‘\( … \)’, except that it does not cause the matched substring to be recorded for future reference.

This is useful when you need a lot of grouping ‘\( … \)’ constructs, but only want to remember one or two – or if you have more than nine groupings and need to use backreferences to refer to the groupings at the end.

Using ‘\(?: … \)’ rather than ‘\( … \)’ when you don’t need the captured substrings ought to speed up your programs some, since it shortens the code path followed by the regular expression engine, as well as the amount of memory allocation and string copying it must do. The actual performance gain to be observed has not been measured or quantified as of this writing.

The shy grouping operator has been borrowed from Perl, and has not been available prior to XEmacs 20.3, nor is it available in FSF Emacs.

\w

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

\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. See Syntax, for a list of syntax codes and the characters that stand for them.

\Scode

matches any character whose syntax is not code.

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.

\`

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 the empty string, but not at the beginning or end of a word.

\<

matches the empty string, but only at the beginning of a word.

\>

matches the empty string, but only at the end of a word.

Here is a complicated regexp used by Emacs to recognize the end of a sentence together with any whitespace that follows. It is given in Lisp syntax to enable you to distinguish the spaces from the tab characters. In Lisp syntax, the string constant begins and ends with a double-quote. ‘\"’ stands for a double-quote as part of the regexp, ‘\\’ for a backslash as part of the regexp, ‘\t’ for a tab and ‘\n’ for a newline.

```"[.?!][]\"')]*\\(\$\\|\t\\|  \\)[ \t\n]*"
```

This regexp contains four parts: a character set matching period, ‘?’ or ‘!’; a character set matching close-brackets, quotes or parentheses, repeated any number of times; an alternative in backslash-parentheses that matches end-of-line, a tab or two spaces; and a character set matching whitespace characters, repeated any number of times.

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