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GnuEmacsRegex.txt
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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 which matches that character and nothing else. The
special characters are `$', `^', `.', `*', `+', `?', `[', `]' and `\'; no
new special characters will be defined. 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 which 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 nontrivial, 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 which begins with `a'
and ends with `b'.
`*'
is not a construct by itself; it is a suffix, which means the
preceding regular expression is to be repeated 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. 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, 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.
`+'
Is a suffix character similar to `*' except that it requires that the
preceding expression be matched at least once. So, for example,
`ca+r' will match the strings `car' and `caaaar' but not the string
`cr', whereas `ca*r' would match all three strings.
`?'
Is a suffix character similar to `*' except that it can match the
preceding expression either once or not at all. For example,
`ca?r' will match `car' or `cr'; nothing else.
`[ ... ]'
`[' begins a "character set", which is terminated by a `]'. In the
simplest case, the characters between the two 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.
Character ranges can also be included in a character set, by writing
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
`$', `%' or period.
Note that the usual special characters are not special any more inside
a character set. A completely different set of special characters
exists inside character sets: `]', `-' and `^'.
To include a `]' in a character set, you must make it the first
character. For example, `[]a]' matches `]' or `a'. To include a `-',
write `---', which is a range containing only `-'. To include `^',
make it other than the first character in the set.
`[^ ... ]'
`[^' begins a "complement character set", which matches any character
except the ones specified. Thus, `[^a-z0-9A-Z]' matches all
characters except letters and digits.
`^' is not special in a character set unless it is the first
character. The character following the `^' is treated as if it
were first (`-' and `]' are not special there).
Note that a complement character set can match a newline, unless
newline is mentioned as one of the characters not to match.
`^'
is a special character that matches the empty string, but only if at
the beginning of a line in the text being matched. Otherwise it fails
to match anything. Thus, `^foo' matches a `foo' which occurs
at the beginning of a line.
`$'
is similar to `^' but matches only at the end of a line. Thus,
`xx*$' matches a string of one `x' or more at the end of a line.
`\'
has two functions: it quotes the special characters (including
`\'), and it introduces additional special constructs.
Because `\' quotes special characters, `\$' is a regular expression
which matches only `$', and `\[' is a regular expression which matches
only `[', and so on.
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; better to quote the special character anyway,
regardless of where is appears.
For the most part, `\' followed by any character matches only
that character. However, there are several exceptions: characters
which, 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
will match.
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 a complicated expression for the postfix `*' to
operate on. Thus, `ba\(na\)*' matches `bananana', etc., with any
(zero or more) number of `na' strings.
3. To mark 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 which 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'
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
mean "match the same text matched the DIGIT'th time by the `\( ... \)'
construct."
The strings matching the first nine `\( ... \)' constructs appearing
in a regular expression are assigned numbers 1 through 9 in order that the
open-parentheses appear in the regular expression. `\1' through
`\9' may be used to refer to the text matched by the corresponding
`\( ... \)' construct.
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.
`\`'
matches the empty string, provided it is at the beginning
of the buffer.
`\''
matches the empty string, provided it is at the end of
the buffer.
`\b'
matches the empty string, provided it is 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, provided it is not at the beginning or
end of a word.
`\<'
matches the empty string, provided it is at the beginning of a word.
`\>'
matches the empty string, provided it is at the end of a word.
`\w'
matches any word-constituent character. The editor syntax table
determines which characters these are.
`\W'
matches any character that is not a word-constituent.
`\sCODE'
matches any character whose syntax is CODE. CODE is a character which
represents a syntax code: thus, `w' for word constituent, `-' for
whitespace, `(' for open-parenthesis, etc. *Note Syntax::.
`\SCODE'
matches any character whose syntax is not CODE.
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 contains four parts in succession: 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.