Reformatting page. Please Wait... done Tcl Tcl Built-In Commands re_syntax(n) ______________________________________________________________________ NAME re_syntax - Syntax of Tcl regular expressions _________________________________________________________________ DESCRIPTION A _r_e_g_u_l_a_r _e_x_p_r_e_s_s_i_o_n 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: _e_x_t_e_n_d_e_d REs (``ERE''s) and _b_a_s_i_c REs (``BRE''s). EREs are roughly those of the traditional _e_g_r_e_p, while BREs are roughly those of the traditional _e_d. This implementation adds a third flavor, _a_d_v_a_n_c_e_d 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 dis- cussed 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 _b_r_a_n_c_h_e_s, separated by ``|'', matching any- thing that matches any of the branches. A branch is zero or more _c_o_n_s_t_r_a_i_n_t_s or _q_u_a_n_t_i_f_i_e_d _a_t_o_m_s, con- catenated. 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 _a_t_o_m possibly followed by a single _q_u_a_n_- _t_i_f_i_e_r. 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}? _n_o_n-_g_r_e_e_d_y quantifiers, which match the same possibili- ties, but prefer the smallest number rather than the largest number of matches (see MATCHING) The forms using { and } are known as _b_o_u_n_ds. The numbers _m and _n are unsigned decimal integers with permissible values from 0 to 255 inclusive. ATOMS An atom is one of: (_r_e) matches a match for _r_e (_r_e is any regular expression) with the match noted for possible reporting (?:_r_e) 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 [_c_h_a_r_s] a _b_r_a_c_k_e_t _e_x_p_r_e_s_s_i_o_n, matching any one of the _c_h_a_r_s (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 char- acters), an _e_s_c_a_p_e (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 _b_o_u_n_d (see above) _x where _x is a single character with no other significance, matches that character. CONSTRAINTS A _c_o_n_s_t_r_a_i_n_t 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 a line9 ar=e]]t'h'e, maenmdbe`r`s[oo]f'' aanreeqaulilvaslyennocneymcoluass.s,Antheeqnui`v`a[l[e=noc=e]]c'l'a,ss``m[a[y=not be an endpoint9 $ matches at the end of a line9 of a range. 9 (?=_r_e) _p_o_s_i_t_i_v_e _l_o_o_k_a_h_e_a_d (AREs only), matches at any point9 (_N_o_t_e: Tcl implements only the Unicode locale. It does not9 where a substring matching _r_e begins9 define any equivalence classes. The examples above are just illustrations.)9 (?!_r_e) _n_e_g_a_t_i_v_e _l_o_o_k_a_h_e_a_d (AREs only), matches at any point where no substring matching _r_e begins The lookahead constraints may not contain back references (see9 ESCAPES9 later), and all parentheses within them are considered non-9 Escapes (AREs only), which begin with a \ followed by an9 capturing.9 alphanumeric character, come in several varieties: character entry, class shorthands, constraint escapes, and back references.9 An RE may not end with ``\''.9 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 charac-9 BRACKET EXPRESSIONS9 ter, and inside a bracket expression, \ is an ordinary character.9 A _b_r_a_c_k_e_t _e_x_p_r_e_s_s_i_o_n is a list of characters enclosed in ``[]''.9 (The latter is the one actual incompatibility between EREs and9 It normally matches any single character from the list (but see9 AREs.)9 below). If the list begins with ``^'', it matches any single character (but see below) _n_o_t from the rest of the list. If two characters in the list are separated by ``-'', this is9 CHARACTER-ENTRY ESCAPES9 shorthand for the full _r_a_n_g_e of characters between those two9 Character-entry escapes (AREs only) exist to make it easier to9 (inclusive) in the collating sequence, e.g. ``[0-9]'' in Unicode9 specify non-printing and otherwise inconvenient characters in9 matches any conventional decimal digit. Two ranges may not share9 REs:9 an endpoint, so e.g. ``a-c-e'' is illegal. Ranges in Tcl always use the Unicode collating sequence, but other programs may use9 \a alert (bell) character, as in C9 other collating sequences and this can be a source of incompata- bility between programs.9 \b backspace, as in C 9 To include a literal ] or - in the list, the simplest method is9 \B synonym for \ to help reduce backslash doubling in some9 to enclose it in [. and .] to make it a collating element (see9 applications where there are multiple levels of backslash9 below). Alternatively, make it the first character (following a9 processing9 possible ``^''), or (AREs only) precede it with ``\''. Alterna- tively, for ``-'', make it the last character, or the second end-9 \c_X (where _X is any character) the character whose low-order 59 point of a range. To use a literal - as the first endpoint of a9 bits are the same as those of _X, and whose other bits are9 range, make it a collating element or (AREs only) precede it with9 all zero9 ``\''. With the exception of these, some combinations using [ (see next paragraphs), and escapes, all other special characters9 \e the character whose collating-sequence name is ``ESC'', or9 lose their special significance within a bracket expression.9 failing that, the character with octal value 033 \f formfeed, as in C 9CHARACTER CLASSES9 \n newline, as in C9 Within a bracket expression, the name of a _c_h_a_r_a_c_t_e_r _c_l_a_s_s enclosed in [: and :] stands for the list of all characters (not9 \r carriage return, as in C9 all collating elements!) belonging to that class. Standard char- acter classes are:9 \t horizontal tab, as in C 9 alpha A letter.9 \u_w_x_y_z (where _w_x_y_z is exactly four hexadecimal digits) the9 upper An upper-case letter.9 Unicode character U+_w_x_y_z in the local byte ordering 9 lower A lower-case letter.9 \U_s_t_u_v_w_x_y_z (where _s_t_u_v_w_x_y_z is exactly eight hexadecimal digits)9 digit A decimal digit.9 reserved for a somewhat-hypothetical Unicode extension to 32 bits9 xdigit A hexadecimal digit. 9 \v vertical tab, as in C are all available.9 alnum An alphanumeric (letter or digit). 9 \x_h_h_h9 print A "printable" (same as graph, except also including9 (where _h_h_h is any sequence of hexadecimal digits) the9 space).9 character whose hexadecimal value is 0x_h_h_h (a single char- acter no matter how many hexadecimal digits are used).9 blank A space or tab character. 9 \0 the character whose value is 09 space A character producing white space in displayed text. 9 \_x_y (where _x_y is exactly two octal digits, and is not a _b_a_c_k9 punct A punctuation character.9 _r_e_f_e_r_e_n_c_e (see below)) the character whose octal value is 0_x_y9 graph A character with a visible representation (includes both alnum and punct).9 \_x_y_z (where _x_y_z is exactly three octal digits, and is not a back reference (see below)) the character whose octal9 cntrl A control character.9 value is 0_x_y_z 9 A locale may provide others. A character class may not be used as9 Hexadecimal digits are ``0''-``9'', ``a''-``f'', and ``A''-``F''.9 an endpoint of a range.9 Octal digits are ``0''-``7''. 9 (_N_o_t_e: the current Tcl implementation has only one locale,9 The character-entry escapes are always taken as ordinary charac-9 the Unicode locale, which supports exactly the above9 ters. For example, \135 is ] in Unicode, but \135 does not ter-9 classes.)9 minate a bracket expression. Beware, however, that some applica- tions (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 see9 BRACKETED CONSTRAINTS9 them, which may require doubling (quadrupling, etc.) the ``\''.9 There are two special cases of bracket expressions: the bracket expressions ``[[:<:]]'' and ``[[:>:]]'' are constraints, match- ing empty strings at the beginning and end of a word respec- tively. A word is defined as a sequence of word characters that9 CLASS-SHORTHAND ESCAPES9 is neither preceded nor followed by word characters. A word char-9 Class-shorthand escapes (AREs only) provide shorthands for cer-9 acter is an _a_l_n_u_m character or an underscore (``_''). These spe-9 tain commonly-used character classes:9 cial bracket expressions are deprecated; users of AREs should use constraint escapes instead (see below).9 \d [[:digit:]] \s [[:space:]] 9COLLATING ELEMENTS9 \w [[:alnum:]_] (note underscore)9 Within a bracket expression, a collating element (a character, a multi-character sequence that collates as if it were a single9 \D [^[:digit:]]9 character, or a collating-sequence name for either) enclosed in [. and .] stands for the sequence of characters of that collating9 \S [^[:space:]]9 element. The sequence is a single element of the bracket expression's list. A bracket expression in a locale that has9 \W [^[:alnum:]_] (note underscore)9 multi-character collating elements can thus match more than one character. So (insidiously), a bracket expression that starts9 Within bracket expressions, ``\d'', ``\s'', and ``\w'' lose their9 with ^ can match multi-character collating elements even if none9 outer brackets, and ``\D'', ``\S'', and ``\W'' are illegal. (So,9 of them appear in the bracket expression!9 for example, ``[a-c\d]'' is equivalent to ``[a-c[:digit:]]''. Also, ``[a-c\D]'', which is equivalent to ``[a-c^[:digit:]]'', is9 (_N_o_t_e: Tcl has no multi-character collating elements. This9 illegal.)9 information is only for illustration.) For example, assume the collating sequence includes a ch multi- character collating element. Then the RE ``[[.ch.]]*c'' (zero or9 CONSTRAINT ESCAPES9 more ``chs'' followed by ``c'') matches the first five characters9 A constraint escape (AREs only) is a constraint, matching the9 of ``chchcc''. Also, the RE ``[^c]b'' matches all of ``chb''9 empty string if specific conditions are met, written as an9 (because ``[^c]'' matches the multi-character ``ch'').9 escape: \A matches only at the beginning of the string (see MATCH- ING, below, for how this differs from ``^'')9 EQUIVALENCE CLASSES Within a bracket expression, a collating element enclosed in [=9 \m matches only at the beginning of a word9 and =] is an equivalence class, standing for the sequences of characters of all collating elements equivalent to that one,9 \M matches only at the end of a word9 including itself. (If there are no other equivalent collating elements, the treatment is as if the enclosing delimiters were9 \y matches only at the beginning or end of a word9 ``[.'' and ``.]''.) For example, if o and 9 \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 _b_a_c_k _r_e_f_e_r_e_n_c_e, see below \_m_n_n (where _m is a nonzero digit, and _n_n is some more digits, and the decimal value _m_n_n is not greater than the number of closing capturing parentheses seen so far) a _b_a_c_k _r_e_f_e_r_e_n_c_e, 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 subexpres- sions. 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 _d_i_r_e_c_t_o_r. 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 _e_m_b_e_d_d_e_d _o_p_t_i_o_n_s: a sequence (?_x_y_z) (where _x_y_z 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 (_t_i_g_h_t) RE syntax, in which all charac- ters are significant, there is an _e_x_p_a_n_d_e_d 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 new- line (or the end of the RE) are ignored, permitting paragraphing and commenting a complex RE. There are three exceptions 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 _s_p_a_c_e character class. Finally, in an ARE, outside bracket expressions, the sequence ``(?#_t_t_t)'' (where _t_t_t 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. _N_o_n_e of these metasyntax extensions is available if the applica- tion (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 _p_r_e_f_e_r_e_n_c_e: 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 shor- test match. A branch has the same preference as the first quanti- fied 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 pos- sible 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 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 charac- ters of ``weeknights'', when ``(.*).*'' is matched against ``abc'' the parenthesized subexpression matches all three charac- ters, 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 expres- sions 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 begin- ning and end of string respectively. ARE \A and \Z continue to match beginning or end of string _o_n_l_y. If partial newline-sensitive matching is specified, this affects 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 seman- tics. 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 _r_e_g_e_x_p package, still in widespread use (e.g., in pre-8.1 releases of Tcl), implemented an early ver- sion of today's EREs. There are four incompatibilities between _r_e_g_e_x_p'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 program- mers 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 parallel, and their perfor- mance is largely insensitive to their complexity) but cases where the search order was exploited to deliberately find a match which was _n_o_t 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 them- selves 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 avail- able, 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 modified 8.1 re_syntax(n) 9