NAME
re_syntax - 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.
DIFFERENT 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 exten
sions.
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 EXPRESSION 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
? a sequence of 0 or 1 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 pos
sibilities, 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-cap
turing'' set of parentheses)
() matches an empty string, noted for possible
reporting
(?:) matches an empty string, without reporting
[chars]
a bracket expression, matching any one of the
chars (see BRACKET EXPRESSIONS 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 fol
lowed by a digit, it is the beginning of a bound
(see above)
x where x is a single character with no other sig
nificance, matches that character.
A constraint matches an empty string when specific
quantifier. 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 consid
ered non-capturing.
An RE may not end with `\'.
BRACKET EXPRESSIONS
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 ASCII matches any decimal digit. Two ranges may
not share an endpoint, so e.g. a-c-e is illegal. Ranges
are very collating-sequence-dependent, and portable pro
grams 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 collat
ing element (see below). Alternatively, make it the first
character (following a possible `^'), or (AREs only) pre
cede it with `\'. Alternatively, for `-', make it the
last character, or the second endpoint of a range. To use
a literal - as the first endpoint of a range, make it a
collating element 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.
Within a bracket expression, a collating element (a char
acter, 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
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 collating elements even if none |
of them appear in the bracket expression! (Note: Tcl cur |
rently has no multi-character collating elements. This |
information is only for illustration.) |
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 equiva
lent 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 syn
onymous. 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.
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
characters that is neither preceded nor followed by word
characters. A word character is an alnum character or an
underscore (_). These special bracket expressions are
deprecated; 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 lat
ter is the one actual incompatibility between EREs and
AREs.)
Character-entry escapes (AREs only) exist to make it eas
ier to specify non-printing and otherwise inconvenient
characters 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
\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
code extension 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 ASCII, 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)
Within bracket expressions, `\d', `\s', and `\w' lose
their outer brackets, and `\D', `\S', and `\W' are ille
gal. (So, for example, [a-c\d] is equivalent to [a- |
c[:digit:]]. Also, [a-c\D], which is equivalent to [a- |
c^[:digit:]], is illegal.)
A constraint escape (AREs only) is a constraint, matching
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 MATCH
ING, 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 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.
application-dependent means. However, this can be over
ridden by a director. If an RE of any flavor begins with
`***:', the rest of the RE is an ARE. If an RE of any
flavor begins with `***=', the rest of the RE is taken to
be a literal string, with all characters considered ordi
nary 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 applica
tion. 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
ordinary characters
s non-newline-sensitive matching (usual default)
t tight syntax (usual default; see below)
w inverse partial newline-sensitive (``weird'') match
ing (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.
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 charac
ters between a # and the following newline (or the end of
the RE) are ignored, permitting paragraphing and comment
ing a complex RE. There are three exceptions to that
basic rule:
retained
white space or `#' within a bracket expression is
retained
white space and comments are illegal within multi-char
acter symbols like the ARE `(?:' or the BRE `\('
Expanded-syntax white-space characters are blank, tab,
newline, and any character that belongs to the space char |
acter 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 sub
string 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 deter
mined by its preference: either the longest substring, or
the shortest.
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 quantified 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 con
sisting of two or more branches connected by the | opera
tor prefers longest match.
Subject to the constraints imposed by the rules for match
ing the whole RE, subexpressions also match the longest or
shortest possible substrings, based on their preferences,
with subexpressions starting earlier in the RE taking pri
ority over ones starting later. Note that outer subex
pressions thus take priority over their component subex
pressions.
force longest and shortest preference, respectively, on a
subexpression or a whole RE.
Match lengths are measured in characters, not collating
elements. An empty string is considered longer than no
match at all. For example, bb* matches the three middle
characters of `abbbc', (week|wee)(night|knights) matches
all ten characters of `weeknights', when (.*).* is
matched against abc the parenthesized subexpression
matches all three characters, and when (a*)* is matched
against bc both the whole RE and the parenthesized subex
pression 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
expression, it is effectively transformed into a bracket
expression containing both cases, so that x becomes
`[xX]'. When it appears inside a bracket expression, all
case counterparts of it are added to the bracket expres
sion, 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 addi
tion to matching at beginning and end of string respec
tively. 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-sensi
tive matching, but not ^ and `$'.
If inverse partial newline-sensitive matching is speci
fied, 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
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 sig
nificance inside bracket expressions. All other ARE fea
tures use syntax which is illegal or has undefined or
unspecified effects in POSIX EREs; the *** syntax of
directors likewise is outside the POSIX syntax for both
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 `\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 restrictions on parentheses and back refer
ences 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 (`RREs' 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 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.
{ followed by a digit in an ARE is the beginning of
a bound, while in RREs, { was always an ordinary
character. Such sequences should be rare, and will
often result in an error because following charac
ters 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 RREs, 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
order. This may affect some RREs which were writ
ten 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 par
allel, 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
BASIC REGULAR EXPRESSIONS
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 possi
ble leading `^'). Finally, single-digit back references
are available, and \< and \> are synonyms for [[:<:]] and
[[:>:]] respectively; no other escapes are available.
SEE ALSO
RegExp(3), regexp(n), regsub(n), lsearch(n), switch(n),
text(n)
KEYWORDS
match, regular expression, string
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