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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 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 sub-
set 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 quanti-
fier, 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 small-
est 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 con-
straints 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 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 incompatabil-
ity 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 con-
straint 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 col-
lating 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 ele-
ment. 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
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 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 refer-
ences. 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 charac-
ter 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-hypo-
thetical 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 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 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:]_] (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 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 subex-
pression 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. Subexpres-
sions 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 miscella-
neous 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 alpha-
betic 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 excep-
tions to that basic rule:
o a white-space character or "#" preceded by "\" is retained
o white space or "#" within a bracket expression is retained
o 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 sub-
string 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 pref-
erence (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.
Note that the quantifiers {1,1} and {1,1}? can be used to force longest and shortest pref-
erence, respectively, on a subexpression or a whole RE.
Match lengths are measured in characters, not collating elements. An empty string is con-
sidered longer than no match at all. For example, "bb*" matches the three middle charac-
ters 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 parenthe-
sized 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 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 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 porta-
ble 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 syn-
tax 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 comple-
mented bracket expressions to the things affected by newline-sensitive matching, the
restrictions on parentheses and back references in lookahead constraints, and the long-
est/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 incompati-
bilities between regexp's near-EREs ("RREs" for short) and AREs. In roughly increasing
order of significance:
o 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.
o { 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.
o 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.
o 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 paral-
lel, 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 long-
est/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 charac-
ter 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 subex-
pression, 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, sin-
gle-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)
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