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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 `\'
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, you can concatenate the regular expressions `f'
and `o' to get the regular expression `fo', which matches only the
string `fo'. To do something nontrivial, you need to use one of the
following special characters:
is a special character that matches any single character except a
newline. Using concatenation, you 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 immediately matching as
many repetitions as it can find. 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. 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.
You can include character ranges 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 `$', `%', or period.
Note that inside a character set the usual special characters are
not special any more. A completely different set of special
characters exists inside character sets: `]', `-', and `^'.
To include a `]' in a character set, you must make it the first
character. For example, `a]' matches `]' or `a'. To include a
`-', write `---', which is a range containing only `-'. To
include `^', make it other than the first character in the set.
`[^ ... ]'
`[^' begins a "complement character set", which matches any
character except the ones specified. Thus, `[^a-z0-9A-Z]' matches
all characters except letters and digits.
`^' is not special in a character set unless it is the first
character. The character following the `^' is treated as if it
were first (`-' and `]' are not special there).
Note that a complement character set can match a newline, unless
newline is mentioned as one of the characters not to match.
is a special character that matches the empty string, but only if
at the beginning of a line in the text being matched. Otherwise,
it fails to match anything. Thus, `^foo' matches a `foo' that
occurs at the beginning of a line.
is similar to `^' but matches only at the end of a line. Thus,
`xx*$' matches a string of one `x' or more at the end of a line.
does two things: 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: 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.
Usually, `\' 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 A or B
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 in practice there is no conflict between the two meanings.
Here is an explanation:
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
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 editor syntax table
determines which characters these are.
matches any character that is not a word-constituent.
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.
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
"[.?!]\"')]*\\($\\|\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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