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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 same
character and nothing else. The special characters are `$',
`^', `.', `*', `+', `?', `[', `]' and
`\'. 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'. (When case distinctions are being ignored, these regexps
also match `F' and `O', but we consider this a generalization
of "the same string", rather than an exception.)
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
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
is not a construct by itself; it is a postfix operator, which means to
match the preceding regular expression repetitively as many times as
possible. Thus, `o*' matches any number of `o's (including no
`*' 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 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 postfix character, 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 character, similar to `*' except that it can match the
preceding expression either once or not at all. For example,
`ca?r' matches `car' or `cr'; nothing else.
- [ ... ]
is a character set, which begins with `[' and is terminated
by `]'. In the simplest case, the characters between the two
brackets are what this set 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), from which it follows that `c[ad]*r'
matches `cr', `car', `cdr', `caddaar', etc.
You can also include character ranges 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 `$', `%' or period.
Note that the usual regexp special characters are not special inside a
character set. A completely different set of special characters exists
inside character sets: `]', `-' and `^'.
To include a `]' in a character set, you must make it the first
character. For example, `a]' matches `]' or `a'. To
include a `-', write `-' as the first or last character of the
set, or put it after a range. Thus, `-]' matches both `]'
To include `^', make it other than the first character in the
- [^ ... ]
`[^' begins a complemented character set, which matches any
character except the ones specified. Thus, `[^a-z0-9A-Z]' matches
all characters except letters and digits.
`^' is not special in a character set unless it is the first
character. The character following the `^' is treated as if it
were first (`-' and `]' are not special there).
A complemented character set can match a newline, unless newline is
mentioned as one of the characters not to match. This is in contrast to
the handling of regexps in programs such as
is a special character that matches the empty string, but only at the
beginning of a line in the text being matched. Otherwise it fails to
match anything. Thus, `^foo' matches a `foo' which occurs at
the beginning of a line.
is similar to `^' but matches only at the end of a line. Thus,
`xx*$' 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 `[', etc.
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: two-character
sequences starting with `\' which have special meanings. The
second character in the sequence is always an ordinary character 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 scope of
Full backtracking capability exists to handle multiple uses of `\|'.
- \( ... \)
is a grouping construct that serves three purposes:
This last application is not a consequence of the idea of a
parenthetical grouping; it is a separate feature which is assigned as a
second meaning to the same `\( ... \)' construct. In practice
there is no conflict between the two meanings. Here is an explanation
of this feature:
To enclose a set of `\|' alternatives for other operations.
Thus, `\(foo\|bar\)x' matches either `foox' or `barx'.
To enclose a complicated expression for the postfix operators `*',
`+' and `?' to operate on. Thus, `ba\(na\)*' matches
`bananana', etc., with any (zero or more) number of `na'
To mark a matched substring for future reference.
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 the
digit d to mean "match the same text matched the dth 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' refer to the text previously 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.
If a particular `\( ... \)' construct matches more than once
(which can easily happen if it is followed by `*'), only the last
match is recorded.
matches the empty string, provided it is at the beginning
of the buffer.
matches the empty string, provided it is at the end of
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 at the beginning or end of the buffer
regardless of what text appears next to it.
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 at the beginning of the buffer only if a
word-constituent character follows.
matches the empty string, provided it is at the end of a word.
`\>' matches at the end of the buffer only if the contents end with
a word-constituent character.
matches any word-constituent character. The syntax table
determines which characters these are. See section The Syntax Table.
matches any character that is not a word-constituent.
matches any character whose syntax is c. Here c is a
character which represents a syntax code: thus, `w' for word
constituent, `(' for open-parenthesis, etc. Represent a character
of whitespace (which can be a newline) by either `-' or a space
matches any character whose syntax is not c.
The constructs that pertain to words and syntax are controlled by the
setting of the syntax table (see section The Syntax Table).
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
"[.?!]\"')]*\\($\\|\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
To enter the same regexp interactively, you would type TAB to
enter a tab, and C-q C-j to enter a newline. You would also type
single slashes as themselves, instead of doubling them for Lisp syntax.
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