re_syntax.n

tcl是工具命令语言

\fB\eS\fR
\fB[^[:space:]]\fR
.TP
\fB\eW\fR
\fB[^[:alnum:]_]\fR
(note underscore)
.RE
.PP
Within bracket expressions, `\fB\ed\fR', `\fB\es\fR',
and `\fB\ew\fR'\&
lose their outer brackets,
and `\fB\eD\fR', `\fB\eS\fR',
and `\fB\eW\fR'\&
are illegal.
.VS 8.2
(So, for example, \fB[a-c\ed]\fR is equivalent to \fB[a-c[:digit:]]\fR.
Also, \fB[a-c\eD]\fR, which is equivalent to \fB[a-c^[:digit:]]\fR, is illegal.)
.VE 8.2
.PP
A constraint escape (AREs only) is a constraint,
matching the empty string if specific conditions are met,
written as an escape:
.RS 2
.TP 6
\fB\eA\fR
matches only at the beginning of the string
(see MATCHING, below, for how this differs from `\fB^\fR')
.TP
\fB\em\fR
matches only at the beginning of a word
.TP
\fB\eM\fR
matches only at the end of a word
.TP
\fB\ey\fR
matches only at the beginning or end of a word
.TP
\fB\eY\fR
matches only at a point that is not the beginning or end of a word
.TP
\fB\eZ\fR
matches only at the end of the string
(see MATCHING, below, for how this differs from `\fB$\fR')
.TP
\fB\e\fIm\fR
(where
\fIm\fR
is a nonzero digit) a \fIback reference\fR, see below
.TP
\fB\e\fImnn\fR
(where
\fIm\fR
is a nonzero digit, and
\fInn\fR
is some more digits,
and the decimal value
\fImnn\fR
is not greater than the number of closing capturing parentheses seen so far)
a \fIback reference\fR, see below
.RE
.PP
A word is defined as in the specification of
\fB[[:<:]]\fR
and
\fB[[:>:]]\fR
above.
Constraint escapes are illegal within bracket expressions.
.PP
A back reference (AREs only) matches the same string matched by the parenthesized
subexpression specified by the number,
so that (e.g.)
\fB([bc])\e1\fR
matches
\fBbb\fR
or
\fBcc\fR
but not `\fBbc\fR'.
The subexpression must entirely precede the back reference in the RE.
Subexpressions are numbered in the order of their leading parentheses.
Non-capturing parentheses do not define subexpressions.
.PP
There is an inherent historical ambiguity between octal character-entry 
escapes and back references, which is resolved by heuristics,
as hinted at above.
A leading zero always indicates an octal escape.
A single non-zero digit, not followed by another digit,
is always taken as a back reference.
A multi-digit sequence not starting with a zero is taken as a back 
reference if it comes after a suitable subexpression
(i.e. the number is in the legal range for a back reference),
and otherwise is taken as octal.
.SH "METASYNTAX"
In addition to the main syntax described above, there are some special
forms and miscellaneous syntactic facilities available.
.PP
Normally the flavor of RE being used is specified by
application-dependent means.
However, this can be overridden by a \fIdirector\fR.
If an RE of any flavor begins with `\fB***:\fR',
the rest of the RE is an ARE.
If an RE of any flavor begins with `\fB***=\fR',
the rest of the RE is taken to be a literal string,
with all characters considered ordinary characters.
.PP
An ARE may begin with \fIembedded options\fR:
a sequence
\fB(?\fIxyz\fB)\fR
(where
\fIxyz\fR
is one or more alphabetic characters)
specifies options affecting the rest of the RE.
These supplement, and can override,
any options specified by the application.
The available option letters are:
.RS 2
.TP 3
\fBb\fR
rest of RE is a BRE
.TP 3
\fBc\fR
case-sensitive matching (usual default)
.TP 3
\fBe\fR
rest of RE is an ERE
.TP 3
\fBi\fR
case-insensitive matching (see MATCHING, below)
.TP 3
\fBm\fR
historical synonym for
\fBn\fR
.TP 3
\fBn\fR
newline-sensitive matching (see MATCHING, below)
.TP 3
\fBp\fR
partial newline-sensitive matching (see MATCHING, below)
.TP 3
\fBq\fR
rest of RE is a literal (``quoted'') string, all ordinary characters
.TP 3
\fBs\fR
non-newline-sensitive matching (usual default)
.TP 3
\fBt\fR
tight syntax (usual default; see below)
.TP 3
\fBw\fR
inverse partial newline-sensitive (``weird'') matching (see MATCHING, below)
.TP 3
\fBx\fR
expanded syntax (see below)
.RE
.PP
Embedded options take effect at the
\fB)\fR
terminating the sequence.
They are available only at the start of an ARE,
and may not be used later within it.
.PP
In addition to the usual (\fItight\fR) RE syntax, in which all characters are
significant, there is an \fIexpanded\fR syntax,
available in all flavors of RE
with the \fB-expanded\fR switch, or in AREs with the embedded x option.
In the expanded syntax,
white-space characters are ignored
and all characters between a
\fB#\fR
and the following newline (or the end of the RE) are ignored,
permitting paragraphing and commenting a complex RE.
There are three exceptions to that basic rule:
.RS 2
.PP
a white-space character or `\fB#\fR' preceded by `\fB\e\fR' is retained
.PP
white space or `\fB#\fR' within a bracket expression is retained
.PP
white space and comments are illegal within multi-character symbols
like the ARE `\fB(?:\fR' or the BRE `\fB\e(\fR'
.RE
.PP
Expanded-syntax white-space characters are blank, tab, newline, and
.VS 8.2
any character that belongs to the \fIspace\fR character class.
.VE 8.2
.PP
Finally, in an ARE,
outside bracket expressions, the sequence `\fB(?#\fIttt\fB)\fR'
(where
\fIttt\fR
is any text not containing a `\fB)\fR')
is a comment,
completely ignored.
Again, this is not allowed between the characters of
multi-character symbols like `\fB(?:\fR'.
Such comments are more a historical artifact than a useful facility,
and their use is deprecated;
use the expanded syntax instead.
.PP
\fINone\fR of these metasyntax extensions is available if the application
(or an initial
\fB***=\fR
director)
has specified that the user's input be treated as a literal string
rather than as an RE.
.SH MATCHING
In the event that an RE could match more than one substring of a given
string,
the RE matches the one starting earliest in the string.
If the RE could match more than one substring starting at that point,
its choice is determined by its \fIpreference\fR:
either the longest substring, or the shortest.
.PP
Most atoms, and all constraints, have no preference.
A parenthesized RE has the same preference (possibly none) as the RE.
A quantified atom with quantifier
\fB{\fIm\fB}\fR
or
\fB{\fIm\fB}?\fR
has the same preference (possibly none) as the atom itself.
A quantified atom with other normal quantifiers (including
\fB{\fIm\fB,\fIn\fB}\fR
with
\fIm\fR
equal to
\fIn\fR)
prefers longest match.
A quantified atom with other non-greedy quantifiers (including
\fB{\fIm\fB,\fIn\fB}?\fR
with
\fIm\fR
equal to
\fIn\fR)
prefers shortest match.
A branch has the same preference as the first quantified atom in it
which has a preference.
An RE consisting of two or more branches connected by the
\fB|\fR
operator prefers longest match.
.PP
Subject to the constraints imposed by the rules for matching the whole RE,
subexpressions also match the longest or shortest possible substrings,
based on their preferences,
with subexpressions starting earlier in the RE taking priority over
ones starting later.
Note that outer subexpressions thus take priority over
their component subexpressions.
.PP
Note that the quantifiers
\fB{1,1}\fR
and
\fB{1,1}?\fR
can be used to force longest and shortest preference, respectively,
on a subexpression or a whole RE.
.PP
Match lengths are measured in characters, not collating elements.
An empty string is considered longer than no match at all.
For example,
\fBbb*\fR
matches the three middle characters of `\fBabbbc\fR',
\fB(week|wee)(night|knights)\fR
matches all ten characters of `\fBweeknights\fR',
when
\fB(.*).*\fR
is matched against
\fBabc\fR
the parenthesized subexpression
matches all three characters, and
when
\fB(a*)*\fR
is matched against
\fBbc\fR
both the whole RE and the parenthesized
subexpression match an empty string.
.PP
If case-independent matching is specified,
the effect is much as if all case distinctions had vanished from the
alphabet.
When an alphabetic that exists in multiple cases appears as an
ordinary character outside a bracket expression, it is effectively
transformed into a bracket expression containing both cases,
so that
\fBx\fR
becomes `\fB[xX]\fR'.
When it appears inside a bracket expression, all case counterparts
of it are added to the bracket expression, so that
\fB[x]\fR
becomes
\fB[xX]\fR
and
\fB[^x]\fR
becomes `\fB[^xX]\fR'.
.PP
If newline-sensitive matching is specified, \fB.\fR
and bracket expressions using
\fB^\fR
will never match the newline character
(so that matches will never cross newlines unless the RE
explicitly arranges it)
and
\fB^\fR
and
\fB$\fR
will match the empty string after and before a newline
respectively, in addition to matching at beginning and end of string
respectively.
ARE
\fB\eA\fR
and
\fB\eZ\fR
continue to match beginning or end of string \fIonly\fR.
.PP
If partial newline-sensitive matching is specified,
this affects \fB.\fR
and bracket expressions
as with newline-sensitive matching, but not
\fB^\fR
and `\fB$\fR'.
.PP
If inverse partial newline-sensitive matching is specified,
this affects
\fB^\fR
and
\fB$\fR
as with
newline-sensitive matching,
but not \fB.\fR
and bracket expressions.
This isn't very useful but is provided for symmetry.
.SH "LIMITS AND COMPATIBILITY"
No particular limit is imposed on the length of REs.
Programs intended to be highly portable should not employ REs longer
than 256 bytes,
as a POSIX-compliant implementation can refuse to accept such REs.
.PP
The only feature of AREs that is actually incompatible with
POSIX EREs is that
\fB\e\fR
does not lose its special
significance inside bracket expressions.
All other ARE features use syntax which is illegal or has
undefined or unspecified effects in POSIX EREs;
the
\fB***\fR
syntax of directors likewise is outside the POSIX
syntax for both BREs and EREs.
.PP
Many of the ARE extensions are borrowed from Perl, but some have
been changed to clean them up, and a few Perl extensions are not present.
Incompatibilities of note include `\fB\eb\fR', `\fB\eB\fR',
the lack of special treatment for a trailing newline,
the addition of complemented bracket expressions to the things
affected by newline-sensitive matching,
the restrictions on parentheses and back references in lookahead constraints,
and the longest/shortest-match (rather than first-match) matching semantics.
.PP
The matching rules for REs containing both normal and non-greedy quantifiers
have changed since early beta-test versions of this package.
(The new rules are much simpler and cleaner,
but don't work as hard at guessing the user's real intentions.)
.PP
Henry Spencer's original 1986 \fIregexp\fR package,
still in widespread use (e.g., in pre-8.1 releases of Tcl),
implemented an early version of today's EREs.
There are four incompatibilities between \fIregexp\fR's near-EREs
(`RREs' for short) and AREs.
In roughly increasing order of significance:
.PP
.RS
In AREs,
\fB\e\fR
followed by an alphanumeric character is either an
escape or an error,
while in RREs, it was just another way of writing the 
alphanumeric.
This should not be a problem because there was no reason to write
such a sequence in RREs.
.PP
\fB{\fR
followed by a digit in an ARE is the beginning of a bound,
while in RREs,
\fB{\fR
was always an ordinary character.
Such sequences should be rare,
and will often result in an error because following characters
will not look like a valid bound.
.PP
In AREs,
\fB\e\fR
remains a special character within `\fB[\|]\fR',
so a literal
\fB\e\fR
within
\fB[\|]\fR
must be written `\fB\e\e\fR'.
\fB\e\e\fR
also gives a literal
\fB\e\fR
within
\fB[\|]\fR
in RREs,
but only truly paranoid programmers routinely doubled the backslash.
.PP
AREs report the longest/shortest match for the RE,
rather than the first found in a specified search order.
This may affect some RREs which were written in the expectation that
the first match would be reported.
(The careful crafting of RREs to optimize the search order for fast
matching is obsolete (AREs examine all possible matches
in parallel, and their performance is largely insensitive to their
complexity) but cases where the search order was exploited to deliberately 
find a match which was \fInot\fR the longest/shortest will need rewriting.)
.RE

.SH "BASIC REGULAR EXPRESSIONS"
BREs differ from EREs in several respects.  `\fB|\fR', `\fB+\fR',
and
\fB?\fR
are ordinary characters and there is no equivalent
for their functionality.
The delimiters for bounds are
\fB\e{\fR
and `\fB\e}\fR',
with
\fB{\fR
and
\fB}\fR
by themselves ordinary characters.
The parentheses for nested subexpressions are
\fB\e(\fR
and `\fB\e)\fR',
with
\fB(\fR
and
\fB)\fR
by themselves ordinary characters.
\fB^\fR
is an ordinary character except at the beginning of the
RE or the beginning of a parenthesized subexpression,
\fB$\fR
is an ordinary character except at the end of the
RE or the end of a parenthesized subexpression,
and
\fB*\fR
is an ordinary character if it appears at the beginning of the
RE or the beginning of a parenthesized subexpression
(after a possible leading `\fB^\fR').
Finally,
single-digit back references are available,
and
\fB\e<\fR
and
\fB\e>\fR
are synonyms for
\fB[[:<:]]\fR
and
\fB[[:>:]]\fR
respectively;
no other escapes are available.

.SH "SEE ALSO"
RegExp(3), regexp(n), regsub(n), lsearch(n), switch(n), text(n)

.SH KEYWORDS
match, regular expression, string