re_syntax.n

tcl是工具命令语言

'\"
'\" Copyright (c) 1998 Sun Microsystems, Inc.
'\" Copyright (c) 1999 Scriptics Corporation
'\"
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'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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'\" RCS: @(#) $Id: re_syntax.n,v 1.3 1999/07/14 19:09:36 jpeek Exp $
'\"
.so man.macros
.TH re_syntax n "8.1" Tcl "Tcl Built-In Commands"
.BS
.SH NAME
re_syntax \- Syntax of Tcl regular expressions.
.BE

.SH DESCRIPTION
.PP
A \fIregular expression\fR describes strings of characters.
It's a pattern that matches certain strings and doesn't match others.

.SH "DIFFERENT FLAVORS OF REs"
Regular expressions (``RE''s), as defined by POSIX, come in two
flavors: \fIextended\fR REs (``EREs'') and \fIbasic\fR REs (``BREs'').
EREs are roughly those of the traditional \fIegrep\fR, while BREs are
roughly those of the traditional \fIed\fR.  This implementation adds
a third flavor, \fIadvanced\fR REs (``AREs''), basically EREs with
some significant extensions.
.PP
This manual page primarily describes AREs.  BREs mostly exist for
backward compatibility in some old programs; they will be discussed at
the end.  POSIX EREs are almost an exact subset of AREs.  Features of
AREs that are not present in EREs will be indicated.

.SH "REGULAR EXPRESSION SYNTAX"
.PP
Tcl regular expressions are implemented using the package written by
Henry Spencer, based on the 1003.2 spec and some (not quite all) of
the Perl5 extensions (thanks, Henry!).  Much of the description of
regular expressions below is copied verbatim from his manual entry.
.PP
An ARE is one or more \fIbranches\fR,
separated by `\fB|\fR',
matching anything that matches any of the branches.
.PP
A branch is zero or more \fIconstraints\fR or \fIquantified atoms\fR,
concatenated.
It matches a match for the first, followed by a match for the second, etc;
an empty branch matches the empty string.
.PP
A quantified atom is an \fIatom\fR possibly followed
by a single \fIquantifier\fR.
Without a quantifier, it matches a match for the atom.
The quantifiers,
and what a so-quantified atom matches, are:
.RS 2
.TP 6
\fB*\fR
a sequence of 0 or more matches of the atom
.TP
\fB+\fR
a sequence of 1 or more matches of the atom
.TP
\fB?\fR
a sequence of 0 or 1 matches of the atom
.TP
\fB{\fIm\fB}\fR
a sequence of exactly \fIm\fR matches of the atom
.TP
\fB{\fIm\fB,}\fR
a sequence of \fIm\fR or more matches of the atom
.TP
\fB{\fIm\fB,\fIn\fB}\fR
a sequence of \fIm\fR through \fIn\fR (inclusive) matches of the atom;
\fIm\fR may not exceed \fIn\fR
.TP
\fB*?  +?  ??  {\fIm\fB}?  {\fIm\fB,}?  {\fIm\fB,\fIn\fB}?\fR
\fInon-greedy\fR quantifiers,
which match the same possibilities,
but prefer the smallest number rather than the largest number
of matches (see MATCHING)
.RE
.PP
The forms using
\fB{\fR and \fB}\fR
are known as \fIbound\fRs.
The numbers
\fIm\fR and \fIn\fR are unsigned decimal integers
with permissible values from 0 to 255 inclusive.
.PP
An atom is one of:
.RS 2
.TP 6
\fB(\fIre\fB)\fR
(where \fIre\fR is any regular expression)
matches a match for
\fIre\fR, with the match noted for possible reporting
.TP
\fB(?:\fIre\fB)\fR
as previous,
but does no reporting
(a ``non-capturing'' set of parentheses)
.TP
\fB()\fR
matches an empty string,
noted for possible reporting
.TP
\fB(?:)\fR
matches an empty string,
without reporting
.TP
\fB[\fIchars\fB]\fR
a \fIbracket expression\fR,
matching any one of the \fIchars\fR (see BRACKET EXPRESSIONS for more detail)
.TP
 \fB.\fR
matches any single character
.TP
\fB\e\fIk\fR
(where \fIk\fR is a non-alphanumeric character)
matches that character taken as an ordinary character,
e.g. \e\e matches a backslash character
.TP
\fB\e\fIc\fR
where \fIc\fR is alphanumeric
(possibly followed by other characters),
an \fIescape\fR (AREs only),
see ESCAPES below
.TP
\fB{\fR
when followed by a character other than a digit,
matches the left-brace character `\fB{\fR';
when followed by a digit, it is the beginning of a
\fIbound\fR (see above)
.TP
\fIx\fR
where \fIx\fR is
a single character with no other significance, matches that character.
.RE
.PP
A \fIconstraint\fR matches an empty string when specific conditions
are met.
A constraint may not be followed by a quantifier.
The simple constraints are as follows; some more constraints are
described later, under ESCAPES.
.RS 2
.TP 8
\fB^\fR
matches at the beginning of a line
.TP
\fB$\fR
matches at the end of a line
.TP
\fB(?=\fIre\fB)\fR
\fIpositive lookahead\fR (AREs only), matches at any point
where a substring matching \fIre\fR begins
.TP
\fB(?!\fIre\fB)\fR
\fInegative lookahead\fR (AREs only), matches at any point
where no substring matching \fIre\fR begins
.RE
.PP
The lookahead constraints may not contain back references (see later),
and all parentheses within them are considered non-capturing.
.PP
An RE may not end with `\fB\e\fR'.

.SH "BRACKET EXPRESSIONS"
A \fIbracket expression\fR is a list of characters enclosed in `\fB[\|]\fR'.
It normally matches any single character from the list (but see below).
If the list begins with `\fB^\fR',
it matches any single character
(but see below) \fInot\fR from the rest of the list.
.PP
If two characters in the list are separated by `\fB\-\fR',
this is shorthand
for the full \fIrange\fR of characters between those two (inclusive) in the
collating sequence,
e.g.
\fB[0\-9]\fR
in ASCII matches any decimal digit.
Two ranges may not share an
endpoint, so e.g.
\fBa\-c\-e\fR
is illegal.
Ranges are very collating-sequence-dependent,
and portable programs should avoid relying on them.
.PP
To include a literal
\fB]\fR
or
\fB\-\fR
in the list,
the simplest method is to
enclose it in
\fB[.\fR and \fB.]\fR
to make it a collating element (see below).
Alternatively,
make it the first character
(following a possible `\fB^\fR'),
or (AREs only) precede it with `\fB\e\fR'.
Alternatively, for `\fB\-\fR',
make it the last character,
or the second endpoint of a range.
To use a literal
\fB\-\fR
as the first endpoint of a range,
make it a collating element
or (AREs only) precede it with `\fB\e\fR'.
With the exception of these, some combinations using
\fB[\fR
(see next
paragraphs), and escapes,
all other special characters lose their
special significance within a bracket expression.
.PP
Within a bracket expression, a collating element (a character,
a multi-character sequence that collates as if it were a single character,
or a collating-sequence name for either)
enclosed in
\fB[.\fR and \fB.]\fR
stands for the
sequence of characters of that collating element.
The sequence is a single element of the bracket expression's list.
A bracket expression in a locale that has
multi-character collating elements
can thus match more than one character.
.VS 8.2
So (insidiously), a bracket expression that starts with \fB^\fR
can match multi-character collating elements even if none of them
appear in the bracket expression!
(\fINote:\fR Tcl currently has no multi-character collating elements.
This information is only for illustration.)
.PP
For example, assume the collating sequence includes a \fBch\fR
multi-character collating element.
Then the RE \fB[[.ch.]]*c\fR (zero or more \fBch\fP's followed by \fBc\fP)
matches the first five characters of `\fBchchcc\fR'.
Also, the RE \fB[^c]b\fR matches all of `\fBchb\fR'
(because \fB[^c]\fR matches the multi-character \fBch\fR).
.VE 8.2
.PP
Within a bracket expression, a collating element enclosed in
\fB[=\fR
and
\fB=]\fR
is an equivalence class, standing for the sequences of characters
of all collating elements equivalent to that one, including itself.
(If there are no other equivalent collating elements,
the treatment is as if the enclosing delimiters were `\fB[.\fR'\&
and `\fB.]\fR'.)
For example, if
\fBo\fR
and
\fB\o'o^'\fR
are the members of an equivalence class,
then `\fB[[=o=]]\fR', `\fB[[=\o'o^'=]]\fR',
and `\fB[o\o'o^']\fR'\&
are all synonymous.
An equivalence class may not be an endpoint
of a range.
.VS 8.2
(\fINote:\fR 
Tcl currently implements only the Unicode locale.
It doesn't define any equivalence classes.
The examples above are just illustrations.)
.VE 8.2
.PP
Within a bracket expression, the name of a \fIcharacter class\fR enclosed
in
\fB[:\fR
and
\fB:]\fR
stands for the list of all characters
(not all collating elements!)
belonging to that
class.
Standard character classes are:
.PP
.RS
.ne 5
.nf
.ta 3c
\fBalpha\fR	A letter. 
\fBupper\fR	An upper-case letter. 
\fBlower\fR	A lower-case letter. 
\fBdigit\fR	A decimal digit. 
\fBxdigit\fR	A hexadecimal digit. 
\fBalnum\fR	An alphanumeric (letter or digit). 
\fBprint\fR	An alphanumeric (same as alnum).
\fBblank\fR	A space or tab character.
\fBspace\fR	A character producing white space in displayed text. 
\fBpunct\fR	A punctuation character. 
\fBgraph\fR	A character with a visible representation. 
\fBcntrl\fR	A control character. 
.fi
.RE
.PP
A locale may provide others.
.VS 8.2
(Note that the current Tcl implementation has only one locale:
the Unicode locale.)
.VE 8.2
A character class may not be used as an endpoint of a range.
.PP
There are two special cases of bracket expressions:
the bracket expressions
\fB[[:<:]]\fR
and
\fB[[:>:]]\fR
are constraints, matching empty strings at
the beginning and end of a word respectively.
'\" note, discussion of escapes below references this definition of word
A word is defined as a sequence of
word characters
that is neither preceded nor followed by
word characters.
A word character is an
\fIalnum\fR
character
or an underscore
(\fB_\fR).
These special bracket expressions are deprecated;
users of AREs should use constraint escapes instead (see below).
.SH ESCAPES
Escapes (AREs only), which begin with a
\fB\e\fR
followed by an alphanumeric character,
come in several varieties:
character entry, class shorthands, constraint escapes, and back references.
A
\fB\e\fR
followed by an alphanumeric character but not constituting
a valid escape is illegal in AREs.
In EREs, there are no escapes:
outside a bracket expression,
a
\fB\e\fR
followed by an alphanumeric character merely stands for that
character as an ordinary character,
and inside a bracket expression,
\fB\e\fR
is an ordinary character.
(The latter is the one actual incompatibility between EREs and AREs.)
.PP
Character-entry escapes (AREs only) exist to make it easier to specify
non-printing and otherwise inconvenient characters in REs:
.RS 2
.TP 5
\fB\ea\fR
alert (bell) character, as in C
.TP
\fB\eb\fR
backspace, as in C
.TP
\fB\eB\fR
synonym for
\fB\e\fR
to help reduce backslash doubling in some
applications where there are multiple levels of backslash processing
.TP
\fB\ec\fIX\fR
(where X is any character) the character whose
low-order 5 bits are the same as those of
\fIX\fR,
and whose other bits are all zero
.TP
\fB\ee\fR
the character whose collating-sequence name
is `\fBESC\fR',
or failing that, the character with octal value 033
.TP
\fB\ef\fR
formfeed, as in C
.TP
\fB\en\fR
newline, as in C
.TP
\fB\er\fR
carriage return, as in C
.TP
\fB\et\fR
horizontal tab, as in C
.TP
\fB\eu\fIwxyz\fR
(where
\fIwxyz\fR
is exactly four hexadecimal digits)
the Unicode character
\fBU+\fIwxyz\fR
in the local byte ordering
.TP
\fB\eU\fIstuvwxyz\fR
(where
\fIstuvwxyz\fR
is exactly eight hexadecimal digits)
reserved for a somewhat-hypothetical Unicode extension to 32 bits
.TP
\fB\ev\fR
vertical tab, as in C
are all available.
.TP
\fB\ex\fIhhh\fR
(where
\fIhhh\fR
is any sequence of hexadecimal digits)
the character whose hexadecimal value is
\fB0x\fIhhh\fR
(a single character no matter how many hexadecimal digits are used).
.TP
\fB\e0\fR
the character whose value is
\fB0\fR
.TP
\fB\e\fIxy\fR
(where
\fIxy\fR
is exactly two octal digits,
and is not a
\fIback reference\fR (see below))
the character whose octal value is
\fB0\fIxy\fR
.TP
\fB\e\fIxyz\fR
(where
\fIxyz\fR
is exactly three octal digits,
and is not a
back reference (see below))
the character whose octal value is
\fB0\fIxyz\fR
.RE
.PP
Hexadecimal digits are `\fB0\fR'-`\fB9\fR', `\fBa\fR'-`\fBf\fR',
and `\fBA\fR'-`\fBF\fR'.
Octal digits are `\fB0\fR'-`\fB7\fR'.
.PP
The character-entry escapes are always taken as ordinary characters.
For example,
\fB\e135\fR
is
\fB]\fR
in ASCII,
but
\fB\e135\fR
does not terminate a bracket expression.
Beware, however, that some applications (e.g., C compilers) interpret 
such sequences themselves before the regular-expression package
gets to see them, which may require doubling (quadrupling, etc.) the `\fB\e\fR'.
.PP
Class-shorthand escapes (AREs only) provide shorthands for certain commonly-used
character classes:
.RS 2
.TP 10
\fB\ed\fR
\fB[[:digit:]]\fR
.TP
\fB\es\fR
\fB[[:space:]]\fR
.TP
\fB\ew\fR
\fB[[:alnum:]_]\fR
(note underscore)
.TP
\fB\eD\fR
\fB[^[:digit:]]\fR
.TP