Tcl Built-In Commands resyntax(1T)
NAME
resyntax - Syntax of Tcl regular expressions.
DESCRIPTION
A regular expression describes strings of characters. It's
a pattern that matches certain strings and doesn't match
others.
DIFERENT FLAVORS OF REs
Regular expressions (``RE''s), as defined by POSIX, come in
two flavors: extended REs (``EREs'') and basic REs
(``BREs''). EREs are roughly those of the traditional
egrep, while BREs are roughly those of the traditional ed.
This implementation adds a third flavor, advanced REs
(``AREs''), basically EREs with some significant extensions.
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.
REGULAR EXPRESION SYNTAX
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.
An ARE is one or more branches, separated by `', matching
anything that matches any of the branches.
A branch is zero or more constraints or quantified atoms,
concatenated. It matches a match for the first, followed by
a match for the second, etc; an empty branch matches the
empty string.
A quantified atom is an atom possibly followed by a single
quantifier. Without a quantifier, it matches a match for
the atom. The quantifiers, and what a so-quantified atom
matches, are:
* a sequence of 0 or more matches of the atom
] a sequence of 1 or more matches of the atom
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? a sequence of 0 or 1 matches of the atom
{m} a sequence of exactly m matches of the atom
{m,} a sequence of m or more matches of the atom
{m,n} a sequence of m through n (inclusive) matches of the
atom; m may not exceed n
*? ]? ?? {m}? {m,}? {m,n}?
non-greedy quantifiers, which match the same possi-
bilities, but prefer the smallest number rather than
the largest number of matches (see MATCHING)
The forms using { and } are known as bounds. The numbers m
and n are unsigned decimal integers with permissible values
from 0 to 255 inclusive.
An atom is one of:
(re) (where re is any regular expression) matches a match
for re, with the match noted for possible reporting
(?:re)
as previous, but does no reporting (a ``non-
capturing'' set of parentheses)
() matches an empty string, noted for possible report-
ing
(?:) matches an empty string, without reporting
[chars]
a bracket expression, matching any one of the chars
(see BRACKET EXPRESIONS for more detail)
. matches any single character
\k (where k is a non-alphanumeric character) matches
that character taken as an ordinary character, e.g.
\\ matches a backslash character
\c where c is alphanumeric (possibly followed by other
characters), an escape (AREs only), see ESCAPES
below
{ when followed by a character other than a digit,
matches the left-brace character `{'; when followed
by a digit, it is the beginning of a bound (see
above)
x where x is a single character with no other
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significance, matches that character.
A constraint matches an empty string when specific condi-
tions are met. A constraint may not be followed by a quan-
tifier. The simple constraints are as follows; some more
constraints are described later, under ESCAPES.
^ matches at the beginning of a line
$ matches at the end of a line
(?=re) positive lookahead (AREs only), matches at any
point where a substring matching re begins
(?!re) negative lookahead (AREs only), matches at any
point where no substring matching re begins
The lookahead constraints may not contain back references
(see later), and all parentheses within them are considered
non-capturing.
An RE may not end with `\'.
BRACKET EXPRESIONS
A bracket expression is a list of characters enclosed in
`[]'. It normally matches any single character from the
list (but see below). If the list begins with `^', it
matches any single character (but see below) not from the
rest of the list.
If two characters in the list are separated by `-', this is
shorthand for the full range of characters between those two
(inclusive) in the collating sequence, e.g. [0-9] in ASCI
matches any decimal digit. Two ranges may not share an end-
point, so e.g. a-c-e is illegal. Ranges are very
collating-sequence-dependent, and portable programs should
avoid relying on them.
To include a literal ] or - in the list, the simplest method
is to enclose it in [. and .] to make it a collating ele-
ment (see below). Alternatively, make it the first charac-
ter (following a possible `^'), or (AREs only) precede it
with `\'. Alternatively, for `-', make it the last charac-
ter, or the second endpoint of a range. To use a literal -
as the first endpoint of a range, make it a collating ele-
ment or (AREs only) precede it with `\'. With the exception
of these, some combinations using [ (see next paragraphs),
and escapes, all other special characters lose their special
significance within a bracket expression.
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Within a bracket expression, a collating element (a charac-
ter, a multi-character sequence that collates as if it were
a single character, or a collating-sequence name for either)
enclosed in [. and .] 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. So (insidiously), a bracket
expression that starts with ^ can match multi-character col-
lating elements even if none of them appear in the bracket
expression! (Note: Tcl currently has no multi-character
collating elements. This information is only for illustra-
tion.)
For example, assume the collating sequence includes a ch
multi-character collating element. Then the RE [.ch.]*c
(zero or more ch's followed by c) matches the first five
characters of `chchcc'. Also, the RE [^c]b matches all of
`chb' (because [^c] matches the multi-character ch).
Within a bracket expression, a collating element enclosed in
[= and =] 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 `[.' and `.]'.) For example, if o
and ^ are the members of an equivalence class, then
`[=o=]', `[=^=]', and `[o^]' are all synonymous. An
equivalence class may not be an endpoint of a range. (Note:
Tcl currently implements only the Unicode locale. It
doesn't define any equivalence classes. The examples above
are just illustrations.)
Within a bracket expression, the name of a character class
enclosed in [: and :] stands for the list of all charac-
ters (not all collating elements!) belonging to that class.
Standard character classes are:
alpha A letter.
upper An upper-case letter.
lower A lower-case letter.
digit A decimal digit.
xdigit A hexadecimal digit.
alnum An alphanumeric (letter or digit).
print An alphanumeric (same as alnum).
blank A space or tab character.
space A character producing white space in displayed text.
punct A punctuation character.
graph A character with a visible representation.
cntrl A control character.
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A locale may provide others. (Note that the current Tcl
implementation has only one locale: the Unicode locale.) A
character class may not be used as an endpoint of a range.
There are two special cases of bracket expressions: the
bracket expressions [:<:] and [:>:] are constraints,
matching empty strings at the beginning and end of a word
respectively. A word is defined as a sequence of word char-
acters that is neither preceded nor followed by word charac-
ters. A word character is an alnum character or an under-
score (). These special bracket expressions are depre-
cated; users of AREs should use constraint escapes instead
(see below).
ESCAPES
Escapes (AREs only), which begin with a \ followed by an
alphanumeric character, come in several varieties: charac-
ter entry, class shorthands, constraint escapes, and back
references. A \ 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
\ followed by an alphanumeric character merely stands for
that character as an ordinary character, and inside a
bracket expression, \ is an ordinary character. (The latter
is the one actual incompatibility between EREs and AREs.)
Character-entry escapes (AREs only) exist to make it easier
to specify non-printing and otherwise inconvenient charac-
ters in REs:
\a alert (bell) character, as in C
\b backspace, as in C
\B synonym for \ to help reduce backslash doubling in
some applications where there are multiple levels of
backslash processing
\cX (where X is any character) the character whose low-
order 5 bits are the same as those of X, and whose
other bits are all zero
\e the character whose collating-sequence name is `ESC',
or failing that, the character with octal value 033
\f formfeed, as in C
\n newline, as in C
\r carriage return, as in C
\t horizontal tab, as in C
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\uwxyz
(where wxyz is exactly four hexadecimal digits) the
Unicode character U]wxyz in the local byte ordering
\Ustuvwxyz
(where stuvwxyz is exactly eight hexadecimal digits)
reserved for a somewhat-hypothetical Unicode exten-
sion to 32 bits
\v vertical tab, as in C are all available.
\xhhh
(where hhh is any sequence of hexadecimal digits) the
character whose hexadecimal value is 0xhhh (a single
character no matter how many hexadecimal digits are
used).
\0 the character whose value is 0
\xy (where xy is exactly two octal digits, and is not a
back reference (see below)) the character whose octal
value is 0xy
\xyz (where xyz is exactly three octal digits, and is not
a back reference (see below)) the character whose
octal value is 0xyz
Hexadecimal digits are `0'-`9', `a'-`f', and `A'-`F'. Octal
digits are `0'-`7'.
The character-entry escapes are always taken as ordinary
characters. For example, \135 is ] in ASCI, but \135 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 `\'.
Class-shorthand escapes (AREs only) provide shorthands for
certain commonly-used character classes:
\d [:digit:]
\s [:space:]
\w [:alnum:] (note underscore)
\D [^[:digit:]
\S [^[:space:]
\W [^[:alnum:] (note underscore)
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Within bracket expressions, `\d', `\s', and `\w' lose their
outer brackets, and `\D', `\S', and `\W' are illegal. (So,
for example, [a-c\d] is equivalent to [a-c[:digit:]. Also,
[a-c\D], which is equivalent to [a-c^[:digit:], is ille-
gal.)
A constraint escape (AREs only) is a constraint, matching
the empty string if specific conditions are met, written as
an escape:
\A matches only at the beginning of the string (see
MATCHING, below, for how this differs from `^')
\m matches only at the beginning of a word
\M matches only at the end of a word
\y matches only at the beginning or end of a word
\Y matches only at a point that is not the beginning or
end of a word
\Z matches only at the end of the string (see MATCHING,
below, for how this differs from `$')
\m (where m is a nonzero digit) a back reference, see
below
\mnn (where m is a nonzero digit, and nn is some more
digits, and the decimal value mnn is not greater
than the number of closing capturing parentheses
seen so far) a back reference, see below
A word is defined as in the specification of [:<:] and
[:>:] above. Constraint escapes are illegal within
bracket expressions.
A back reference (AREs only) matches the same string matched
by the parenthesized subexpression specified by the number,
so that (e.g.) ([bc])\1 matches bb or cc but not `bc'. 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.
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
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subexpression (i.e. the number is in the legal range for a
back reference), and otherwise is taken as octal.
METASYNTAX
In addition to the main syntax described above, there are
some special forms and miscellaneous syntactic facilities
available.
Normally the flavor of RE being used is specified by
application-dependent means. However, this can be overrid-
den by a director. If an RE of any flavor begins with
`***:', the rest of the RE is an ARE. If an RE of any fla-
vor begins with `***=', the rest of the RE is taken to be a
literal string, with all characters considered ordinary
characters.
An ARE may begin with embedded options: a sequence (?xyz)
(where xyz 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:
b rest of RE is a BRE
c case-sensitive matching (usual default)
e rest of RE is an ERE
i case-insensitive matching (see MATCHING, below)
m historical synonym for n
n newline-sensitive matching (see MATCHING, below)
p partial newline-sensitive matching (see MATCHING,
below)
q rest of RE is a literal (``quoted'') string, all ordi-
nary characters
s non-newline-sensitive matching (usual default)
t tight syntax (usual default; see below)
w inverse partial newline-sensitive (``weird'') matching
(see MATCHING, below)
x expanded syntax (see below)
Embedded options take effect at the ) terminating the
sequence. They are available only at the start of an ARE,
and may not be used later within it.
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In addition to the usual (tight) RE syntax, in which all
characters are significant, there is an expanded syntax,
available in all flavors of RE with the -expanded switch, or
in AREs with the embedded x option. In the expanded syntax,
white-space characters are ignored and all characters
between a # and the following newline (or the end of the RE)
are ignored, permitting paragraphing and commenting a com-
plex RE. There are three exceptions to that basic rule:
a white-space character or `#' preceded by `\' is retained
white space or `#' within a bracket expression is retained
white space and comments are illegal within multi-
character symbols like the ARE `(?:' or the BRE `\('
Expanded-syntax white-space characters are blank, tab, new-
line, and any character that belongs to the space character
class.
Finally, in an ARE, outside bracket expressions, the
sequence `(?#ttt)' (where ttt is any text not containing a
`)') is a comment, completely ignored. Again, this is not
allowed between the characters of multi-character symbols
like `(?:'. Such comments are more a historical artifact
than a useful facility, and their use is deprecated; use the
expanded syntax instead.
None of these metasyntax extensions is available if the
application (or an initial ***= director) has specified that
the user's input be treated as a literal string rather than
as an RE.
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 sub-
string starting at that point, its choice is determined by
its preference: either the longest substring, or the shor-
test.
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 {m} or {m}? has
the same preference (possibly none) as the atom itself. A
quantified atom with other normal quantifiers (including
{m,n} with m equal to n) prefers longest match. A quanti-
fied atom with other non-greedy quantifiers (including
{m,n}? with m equal to n) 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 operator prefers longest match.
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Subject to the constraints imposed by the rules for matching
the whole RE, subexpressions also match the longest or shor-
test 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.
Note that the quantifiers {1,1} and {1,1}? can be used to
force longest and shortest preference, respectively, on a
subexpression or a whole RE.
Match lengths are measured in characters, not collating ele-
ments. An empty string is considered longer than no match
at all. For example, bb* matches the three middle charac-
ters of `abbbc', (weekwee)(nightknights) matches all ten
characters of `weeknights', when (.*).* is matched against
abc the parenthesized subexpression matches all three char-
acters, and when (a*)* is matched against bc both the whole
RE and the parenthesized subexpression match an empty
string.
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 expres-
sion, it is effectively transformed into a bracket expres-
sion containing both cases, so that x becomes `[xX]'. When
it appears inside a bracket expression, all case counter-
parts of it are added to the bracket expression, so that [x]
becomes [xX] and [^x] becomes `[^xX]'.
If newline-sensitive matching is specified, . and bracket
expressions using ^ will never match the newline character
(so that matches will never cross newlines unless the RE
explicitly arranges it) and ^ and $ will match the empty
string after and before a newline respectively, in addition
to matching at beginning and end of string respectively.
ARE \A and \Z continue to match beginning or end of string
only.
If partial newline-sensitive matching is specified, this
affects . and bracket expressions as with newline-sensitive
matching, but not ^ and `$'.
If inverse partial newline-sensitive matching is specified,
this affects ^ and $ as with newline-sensitive matching, but
not . and bracket expressions. This isn't very useful but
is provided for symmetry.
LIMITS AND COMPATIBILITY
No particular limit is imposed on the length of REs. Pro-
grams intended to be highly portable should not employ REs
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longer than 256 bytes, as a POSIX-compliant implementation
can refuse to accept such REs.
The only feature of AREs that is actually incompatible with
POSIX EREs is that \ does not lose its special significance
inside bracket expressions. All other ARE features use syn-
tax which is illegal or has undefined or unspecified effects
in POSIX EREs; the *** syntax of directors likewise is out-
side the POSIX syntax for both BREs and EREs.
Many of the ARE extensions are borrowed from Perl, but some
have been changed to clean them up, and a few Perl exten-
sions are not present. Incompatibilities of note include
`\b', `\B', the lack of special treatment for a trailing
newline, the addition of complemented bracket expressions to
the things affected by newline-sensitive matching, the res-
trictions on parentheses and back references in lookahead
constraints, and the longest/shortest-match (rather than
first-match) matching semantics.
The matching rules for REs containing both normal and non-
greedy quantifiers have changed since early beta-test ver-
sions of this package. (The new rules are much simpler and
cleaner, but don't work as hard at guessing the user's real
intentions.)
Henry Spencer's original 1986 regexp package, still in
widespread use (e.g., in pre-8.1 releases of Tcl), imple-
mented an early version of today's EREs. There are four
incompatibilities between regexp's near-EREs (`REs' for
short) and AREs. In roughly increasing order of signifi-
cance:
In AREs, \ followed by an alphanumeric character is
either an escape or an error, while in REs, 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 REs.
{ followed by a digit in an ARE is the beginning of a
bound, while in REs, { was always an ordinary charac-
ter. Such sequences should be rare, and will often
result in an error because following characters will
not look like a valid bound.
In AREs, \ remains a special character within `[]', so
a literal \ within [] must be written `\\'. \\ also
gives a literal \ within [] in REs, but only truly
paranoid programmers routinely doubled the backslash.
AREs report the longest/shortest match for the RE,
rather than the first found in a specified search
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order. This may affect some REs which were written in
the expectation that the first match would be reported.
(The careful crafting of REs 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 not the longest/shortest will
need rewriting.)
BASIC REGULAR EXPRESIONS
BREs differ from EREs in several respects. `', `]', and ?
are ordinary characters and there is no equivalent for their
functionality. The delimiters for bounds are \{ and `\}',
with { and } by themselves ordinary characters. The
parentheses for nested subexpressions are \( and `\)', with
( and ) by themselves ordinary characters. ^ is an ordinary
character except at the beginning of the RE or the beginning
of a parenthesized subexpression, $ is an ordinary character
except at the end of the RE or the end of a parenthesized
subexpression, and * is an ordinary character if it appears
at the beginning of the RE or the beginning of a
parenthesized subexpression (after a possible leading `^').
Finally, single-digit back references are available, and \<
and \> are synonyms for [:<:] and [:>:] respectively;
no other escapes are available.
SEE ALSO
RegExp(3TCL), regexp(1T), regsub(1T), lsearch(1T),
switch(1T), text(1T)
KEYWORDS
match, regular expression, string
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
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ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWTcl
Interface Stability Uncommitted
NOTES
Source for Tcl is available on http:/opensolaris.org.
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