A regular expression describes strings of characters. It's a pattern that matches certain strings and does not match others.
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.
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.
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.
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 “\”
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.
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.)
(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”
(Note: Tcl implements only the Unicode locale. It does not define any equivalence classes. The examples above are just illustrations.)
Hexadecimal digits are .QR 0 9 , .QR a f , and .QR A F . Octal digits are .QR 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 “\”
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.)
A word is defined as in the specification of “[[:<:]]” and “[[:>:]]” above. Constraint escapes are illegal within bracket expressions.
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.
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:
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:
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.
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.
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”) and AREs. In roughly increasing order of significance: