re_syntax
re_syntax(n) Tcl Built-In Commands re_syntax(n)
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NAME
re_syntax - Syntax of Tcl regular expressions.
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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 fla-
vors: 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 back-
ward 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 quanti-
fiers, 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 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 possibilities, 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 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 charac-
ter taken as an ordinary character, e.g. \\ matches a backslash
character
\c where c is alphanumeric (possibly followed by other charac-
ters), 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 significance,
matches that character.
A constraint matches an empty string when specific conditions are met.
A constraint may not be followed by a quantifier. The simple con-
straints 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 EXPRESSIONS
A bracket expression is a list of characters enclosed in `[]'. It nor-
mally 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 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 element (see below).
Alternatively, make it the first character (following a possible `^'),
or (AREs only) precede it with `\'. Alternatively, for `-', make it
the last character, or the second endpoint of a range. To use a lit-
eral - 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 spe-
cial characters lose their special significance within a bracket
expression.
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 [. 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 expres-
sion 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 currently |
has no multi-character collating elements. This information is only |
for illustration.) |
For example, assume the collating sequence includes a ch multi-charac- |
ter collating element. Then the RE [[.ch.]]*c (zero or more ch's fol- |
lowed 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 end-
point 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 characters (not all collating ele-
ments!) 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 expres-
sions [[:<:]] and [[:>:]] are constraints, matching empty strings at
the beginning and end of a word respectively. A word is defined as a
sequence of word characters that is neither preceded nor followed by
word characters. A word character is an alnum character or an under-
score (_). 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: character entry, class short-
hands, 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 char-
acter 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 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 applica-
tions 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 digits) reserved
for a somewhat-hypothetical Unicode 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 refer-
ence (see below)) the character whose octal value is 0xy
\xyz (where xyz is exactly three octal digits, and is not a back ref-
erence (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 compil-
ers) 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 com-
monly-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 illegal. (So, for example, [a- |
c\d] is equivalent to [a-c[:digit:]]. Also, [a-c\D], which is equiva- |
lent to [a-c^[:digit:]], is illegal.)
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 num-
bered in the order of their leading parentheses. Non-capturing paren-
theses 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 oth-
erwise 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-depen-
dent means. However, this can be overridden 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 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 ordinary charac-
ters
s non-newline-sensitive matching (usual default)
t tight syntax (usual default; see below)
w inverse partial newline-sensitive (``weird'') matching (see MATCH-
ING, 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
characters between a # 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:
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, newline, 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 substring starting at that point, its
choice is determined 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 quanti-
fiers (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 pref-
erence 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.
Subject to the constraints imposed by the rules for matching the whole
RE, subexpressions also match the longest or shortest possible sub-
strings, based on their preferences, with subexpressions starting ear-
lier in the RE taking priority over ones starting later. Note that
outer subexpressions thus take priority over their component subexpres-
sions.
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 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 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 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 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 symme-
try.
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.
The only feature of AREs that is actually incompatible with POSIX EREs
is that \ does not lose its special significance inside bracket expres-
sions. All other ARE features use syntax which is illegal or has unde-
fined or unspecified effects in POSIX EREs; the *** syntax of directors
likewise is outside 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 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 references in lookahead con-
straints, and the longest/shortest-match (rather than first-match)
matching semantics.
The matching rules for REs containing both normal and non-greedy quan-
tifiers 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.)
Henry Spencer's original 1986 regexp 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 regexp's near-
EREs (`RREs' for short) and AREs. In roughly increasing order of sig-
nificance:
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 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 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 written in the expectation that the first
match would be reported. (The careful crafting of RREs to opti-
mize the search order for fast matching is obsolete (AREs exam-
ine 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 EXPRESSIONS
BREs differ from EREs in several respects. `|', `+', and ? are ordi-
nary 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(3), regexp(n), regsub(n), lsearch(n), switch(n), text(n)
KEYWORDS
match, regular expression, string
Tcl 8.1 re_syntax(n)
