Provided by: tcl9.0-doc_9.0.1+dfsg-1_all 
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
does not match others.
DIFFERENT FLAVORS OF REs
Regular expressions (“RE”s), as defined by POSIX, come in two flavors: extended REs (“ERE”s) and basic
REs (“BRE”s). 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 (“ARE”s), 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 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.
QUANTIFIERS
A quantified atom is an atom possibly followed by a single quantifier. Without a quantifier, it matches
a single 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 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.
ATOMS
An atom is one of:
(re) matches a match for re (re is any regular expression) 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 matches the non-alphanumeric character k 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 significance, matches that character.
CONSTRAINTS
A constraint 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.
^ matches at the beginning of the string or a line (according to whether matching is newline-
sensitive or not, as described in MATCHING, below).
$ matches at the end of the string or a line (according to whether matching is newline-sensitive
or not, as described in MATCHING, below).
The difference between string and line matching modes is immaterial when the string does not
contain a newline character. The \A and \Z constraint escapes have a similar purpose but are
always constraints for the overall string.
The default newline-sensitivity depends on the command that uses the regular expression, and
can be overridden as described in METASYNTAX, below.
(?=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 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 Unicode matches any
conventional decimal digit. Two ranges may not share an endpoint, so e.g. “a-c-e” is illegal. Ranges in
Tcl always use the Unicode collating sequence, but other programs may use other collating sequences and
this can be a source of incompatibility between programs.
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 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.
CHARACTER CLASSES
Within a bracket expression, the name of a character class enclosed in [: and :] stands for the list of
all characters (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 A "printable" (same as graph, except also including space).
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 (includes both alnum and punct).
cntrl A control character.
A locale may provide others. A character class may not be used as an endpoint of a range.
(Note: the current Tcl implementation has only one locale, the Unicode locale, which supports
exactly the above classes.)
BRACKETED CONSTRAINTS
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 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).
COLLATING ELEMENTS
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 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 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 “chs” 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”).
EQUIVALENCE CLASSES
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 ô are the members of an equivalence class, then “[[=o=]]”,
“[[=ô=]]”, and “[oô]” are all synonymous. An equivalence class may not be an endpoint of a range.
(Note: Tcl implements only the Unicode locale. It does not define any equivalence classes. The
examples above are just illustrations.)
ESCAPES
Escapes (AREs only), which begin with a \ followed by an alphanumeric character, come in several
varieties: character 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
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 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 one up to four hexadecimal digits) the Unicode character U+wxyz in the local byte
ordering
\Ustuvwxyz
(where stuvwxyz is one up to eight hexadecimal digits) reserved for a Unicode extension up to 21
bits. The digits are parsed until the first non-hexadecimal character is encountered, the maximum
of eight hexadecimal digits are reached, or an overflow would occur in the maximum value of
U+10ffff.
\v vertical tab, as in C
\xhh (where hh is one or two hexadecimal digits) the character whose hexadecimal value is 0xhh.
\0 the character whose value is 0
\xyz (where xyz is exactly three octal digits, and is not a back reference (see below)) the character
whose octal value is 0xyz. The first digit must be in the range 0-3, otherwise the two-digit form
is assumed.
\xy (where xy is exactly two octal digits, and is not a back reference (see below)) the character
whose octal value is 0xy
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 Unicode,
but \135 does not terminate a bracket expression. Beware, however, that some applications (e.g., C
compilers and the Tcl interpreter if the regular expression is not quoted with braces) interpret such
sequences themselves before the regular-expression package gets to see them, which may require doubling
(quadrupling, etc.) the “\”.
CLASS-SHORTHAND ESCAPES
Class-shorthand escapes (AREs only) provide shorthands for certain commonly-used character classes:
\d [[:digit:]]
\s [[:space:]]
\w [[:alnum:]_\u203F\u2040\u2054\uFE33\uFE34\uFE4D\uFE4E\uFE4F\uFF3F] (including punctuation
connector characters)
\D [^[:digit:]]
\S [^[:space:]]
\W [^[:alnum:]_\u203F\u2040\u2054\uFE33\uFE34\uFE4D\uFE4E\uFE4F\uFF3F] (including punctuation
connector characters)
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 illegal.)
CONSTRAINT ESCAPES
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.
BACK REFERENCES
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.
Normally the flavor of RE being used is specified by application-dependent 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 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.
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 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 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 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.
The quantifiers {1,1} and {1,1}? can be used to force longest and shortest preference, respectively, on a
subexpression or a whole RE.
NOTE: This means that you can usually make a RE be non-greedy overall by putting {1,1}? after one
of the first non-constraint atoms or parenthesized sub-expressions in it. It pays to experiment
with the placing of this non-greediness override on a suitable range of input texts when you are
writing a RE if you are using this level of complexity.
For example, this regular expression is non-greedy, and will match the shortest substring possible
given that “abc” will be matched as early as possible (the quantifier does not change that):
ab{1,1}?c.*x.*cba
The atom “a” has no greediness preference, we explicitly give one for “b”, and the remaining
quantifiers are overridden to be non-greedy by the preceding non-greedy quantifier.
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 is not very useful but is provided for
symmetry.
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 expressions. All other ARE features 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 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
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 versions of this package. (The new rules are much simpler and cleaner, but do not 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 significance:
• 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 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 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 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(3tcl), regsub(3tcl), lsearch(3tcl), switch(3tcl), text(3tk)
KEYWORDS
match, regular expression, string
Tcl 8.1 re_syntax(3tcl)