Class regex
implements regular-expression handling. A regular
expression is a specification of a string with various wildcard
mechanisms. It is compiled to a finite state machine which is executed
to make the match. XPCE's regular expression use Henry Spencer's regex
library, modified by the Tcl project to handle UNICODE and modified by
us to deal with implementation details of class text_buffer
and class char_array.
Regular expressions can be used to search in one of the following
objects:
<-ignore_case If @on,
the match is done ignoring case. The regex library uses the UNICODE
locale for case matching Default is to match case-senstive.
<-syntax
One of basic, extended or advanced. The default is advanced. See regex->pattern
for details.
<-pattern
Represents the actual pattern. The syntax of regular expressions is
described with regex<->pattern.
The following code searched for method references in the PCE documentation style.
find_method_ref(TextBuffer, Access, Method) :-
new(R, regex('(->|<-|<->)(\w+)')),
send(R, search, TextBuffer),
get(R, register_value,
1, TextBuffer, name, Arrow),
get(R, register_value,
2, TextBuffer, name, Method),
map_arrow(Arrow, Access).
map_arrow(->, send).
map_arrow(<-, get).
map_arrow(<->, both).
A regex object
holds information about the last match. This information will be
overwritten on a new regex->match
or regex->search.
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 flavors: 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 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.
A quantified atom is an atom possibly followed by a single quantifier. Without a quantifier, it matches a match for the atom. The quantifiers, and what a so-quantified atom matches, are:
? 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 { a nd } 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:
() matches an empty string, noted for possible reporting
(?:) matches an empty string, without reporting
. matches any single character
\k (where k is a non-alphanumeric character) matches that
character taken as an ordinary character, e.g. \\
matches a backslash charac‐ ter
\c where c is alphanumeric (possibly followed by other
characters), an escape (AREs only), see ESCAPES below
A constraint ma tches 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 line
$ matches at the end of a line
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
ASCII matches any decimal digit. Two ranges may not share an endpoint,
so e.g. a-c-e is illegal. Ranges are very
collat‐ ing-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 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.
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 currently 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 ch's 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).
Within a bracket expression, a collating element enclosed in [= and
=] is an equivalence class, standing for the sequences of characters of
all col‐ lating 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 synony‐ mous. An equivalence
class may not be an endpoint 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 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). 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 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).
ESCAPES
Escapes (AREs only), which begin with a \ followed by an
alphanumeric char‐ acter, 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 (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 exactly four hexadecimal digits) the
Unicode charac‐ ter 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 reference (see below)) the character whose octal value is 0xy
\xyz (where xyz is exactly three octal digits, and is not a
back refer‐ ence (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 expres‐ sion. Beware, however, that some
applications (e.g., C compilers) inter‐
pret 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 equivalent 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 captur‐ ing 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 numbered in the order of their leading parentheses.
Non-capturing parentheses do not define subex‐ pressions.
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)
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.
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
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-char‐ acter symbols like ‘(?:’. Such comments are more a historical artifact than a useful facility, and their use is deprecated; use the expanded syn‐ tax 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 short‐ est.
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 short‐ est 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 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 con‐ taining 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 con‐
tinue 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
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 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 significance:
\ 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.
\ 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.
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 [[:>:]]
respec‐ tively; no other escapes are available.
<-pattern
for a description of the pattern and how it is affected by this
variable. See also
regex->ignore_case.
<->pattern
to a compiled representation in _-compiled_. Normally invoked
automatically by regex->search
when needed.
If the argument is @on,
the pattern is compiled optimised, increasing compilation
time, but reducing search time.
file_pattern represents a common (Unix csh)
file-pattern, translate it into a regular expression representing the
same pattern and associate this using regex->pattern.
The following constructs are mapped:
? .
* .*
[...] [...]
{a,b} \(a\|b\)
The pattern is closed with a’$’sign.
|text_buffer|fragment,
action=code, from=[int], to=[int]
@arg1 The regex object @arg2 The object searched in
After each successful regex->search,
the argument code object
is executed. The next regex->search
is started at regex<-register_end.
If the regex matched an empty string and the string is still empty after
executing the code
object, the search is restarted at
regex<-register_end
+ 1 to avoid a loop.
When from and to are specified, the for_all
is ran only in the specified range.
The code executed may invoke regex->register_value, regex->replace,
etc. to modify the text. The following code replaces all foo
in
bar in string S:
?- send(regex(foo), for_all, S,
message(@arg1, replace, @arg2, bar)).
See also char_array<-split.
->search
when @on.->compile’d
on the first call to regex->match
or
regex->search.
After creation the pattern may be changed using
regex->pattern.
If the case_sensitive argument is @off,
searching and matching is caried out ignoring case. See also regex->ignore_case.
|text_buffer|fragment,
start=[int], end=[int]<-match-patternregex->replace
is equivalent to regex->register_value:
Object, Value, but in addition to this method expands the special
construct’\digit’with the text of the
corresponding register value.
|text_buffer|fragment,
start=[int], end=[int]<-register_start, regex<-register_end
and
regex<-register_value
may be used to get information on the location of the match.
If end is smaller than start, the search is
executed backwards. The regex<-search
variant returns the regex<-register_start.
See also regex->pattern, regex->match
and regex<-match.
|text_buffer|fragment,
start=[int], end=[int] -> length=int<-pattern
with the given object. The match starts at start (default
0) and should not pass end (default end of the object
matched). The regex<-match
variant returns the number of characters matched. For both methods, regex<-register_start,
regex<-register_value,
etc. may be used afterwards.
See also regex->search
and regex->match.
?- get(regex(''), quote, '^hello*', S),
get(S, value, T).
S = @27463123
T = '\^hello\*'
|text_buffer|fragment,
start=[int], end=[int] -> start=int<-register_start, regex<-register_end
and
regex<-register_value
may be used to get information on the location of the match.
If end is smaller than start, the search is
executed backwards. The regex<-search
variant returns the regex<-register_start.
Inherits description from: regex->search