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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 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
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.
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 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 `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 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 a
`]'. 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 `$', `%' or period.
Note that the usual special characters are not special any more
inside a character set. A completely different set of special
characters exists inside character sets: `]', `-' and `^'.
To include a `]' in a character set, you must make it the first
character. For example, `a]' matches `]' or `a'. To include a
`-', write `-' at the beginning or end of a range. To include
`^', make it other than the first character in the set.
`[^ ... ]'
`[^' begins a "complemented character set", which matches any
character except the ones specified. Thus, `[^a-z0-9A-Z]' matches
all characters *except* letters and digits.
`^' is not special in a character set unless it is the first
character. The character following the `^' is treated as if it
were first (`-' and `]' are not special there).
A complemented character set can match a newline, unless newline is
mentioned as one of the characters not to match. This is in
contrast to the handling of regexps in programs such as `grep'.
is a special character that matches the empty string, but only at
the beginning of a line in the text being matched. Otherwise it
fails to match anything. Thus, `^foo' matches a `foo' which
occurs at the beginning of a line.
is similar to `^' but matches only at the end of a line. Thus,
`xx*$' 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:
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 operators
`*', `+' and `?' to operate on. Thus, `ba\(na\)*' matches
`bananana', etc., with any (zero or more) number of `na'
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 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:
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
matches the empty string, provided it is at the end of the buffer.
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
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
matches the empty string, provided it is at the end of a word.
matches any word-constituent character. The syntax table
determines which characters these are.
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 (Note: Syntax.).
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
"[.?!]\"')]*\\($\\|\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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