Squeak
  links to this page:    
View this PageEdit this PageUploads to this PageHistory of this PageTop of the SwikiRecent ChangesSearch the SwikiHelp Guide
Regular Expressions in Squeak - Perl-Style
Last updated at 3:53 pm UTC on 9 March 2009

Perl-compatible Regular Expressions for Squeak: a Plugin for Philip Hazel's PCRE


RegularExpressionPlugin can be loaded from SM.
How to install the RePlugin .SAR from SqueakMap on Unix

I. Introduction


RePlugin is a Smalltalk implementation of modern Perl-Style regular expressions written by Andrew C. Greenberg , based upon the excellent PCRE library by Philip Hazel. As discussed in RePattern aGeneralComment, the functionality is essentially embodied in this class, Class ReMatch and certain new messages in Class String. A summary of the regular expression syntax can be found in RePattern aRegexComment and a summary of the compile option codes available can be found in RePattern anOptionsComment.

This file describes the regular expression structure recognized by RePlugin and the accompanying smalltalk wrapper, Substantial portions are excerpted directly from the documentation in Philip Hazel's excellent PCRE package. All errors are mine.

Overview of the 'Package.'

The following classes are provided:


RePattern Objects Represented Compiled RE's for Repeated Matching
ReMatch Objects Representing the Results of a Successful Search
RePlugin The Plugin 'Glue' to the PCRE Library.


There is a utility class, PluginCodeGenerator, which is used to facilitate the generation of the plugin. Additionally, several methods were added to the String class for convenience, all located under the browser category 'RePattern support.' These include methods permitting creation of re objects and match objects with expressions such as the following:


'andy' asRe
'.andy' reSearch: 'This string is dandy like candy!'


Ordinarily, one compiles a regular expression into a RePattern object, which can then be matched against subsequent strings, as follows:


|re m|
re _ '(\d\d)/(\d\d)/((19|20)?\d\d)' asRe.
m _ re search: 'Andy was born on 10/02/1957, and not soon enough!'.
m matches
('10' '02' '1957' '19' )


The first message answers the compiled expression, encapsulated in RePattern re. The second message answers a ReMatch m representing the result of a search of the string for matches of re (nil would be returned if no match was found). The third message answered a collection of the parenthetical subgroups matched, each showing the day, month and year as extracted from the string.

Further examples and documentation can be found in the comments and definitions set forth in ReMatch, RePattern and String

Using RePattern To Create Compiled Regular Expression Objects (For Repeated Matching):


'^Squ(ea|aw)k!$' asRe
'^Squ(ea|aw)k!$' asReOpt: 'imsxAEX'
'^Squ(ea|aw)k!$' asReOpt: 'imsxAEX' onErrorRun: aBlock

RePattern on: '^Squ(ea|aw)k!$'
RePattern on: '^Squ(ea|aw)k!$' opt: 'imsxAEX'
RePattern
on: '^Squ(ea|aw)k!$'
opt: 'imsxAEX'
onErrorRun: [:pat :offset :message | "your code here"


Each of the preceding expressions returns an instance of RePattern, compiled for efficient subsequent matching. To perform a one-time search, see the messages defined in ReMatch.

Using RePattern To Search a Compiled Regexp Against A String or Substring for Matches:


re search: 'Squeak or Squawk!'.
re search: 'Squeak or Squawk!' from: 11.
re search: 'Squeak or Squawk!' from: 11 to: 16.
re search: 'Squeak or Squawk!' opt: 'ABZ'.
re search: 'Squeak or Squawk!' from: 11 opt: 'ABZ'.
re search: 'Squeak or Squawk!' from: 11 to: 16 opt: 'ABZ'.
re search: 'Squeak or Squawk!' opt: 'ABZ'.
re search: 'Squeak or Squawk!' from: 11 to: 16 opt: 'ABZ'.


If no match, these messages answer nil. Otherwise, they answer with a corresponding instance of ReMatch.

Using RePattern for Global Searching and Replacing

The re package provides rudimentary facilities for global searches and replacements on a string. The following expressions


'\w+' reGsearch: 'this is a test'
(RePattern on: '\w+') gsearch: 'this is a test'


return an ordered collection of the results of repeated non-overlapping applications of the pattern to the string, or nil if there are no matches in the string. To produce a list of matched strings, you can for example apply the 'collect:' message:


('\w+' reGsearch: 'this is a test') collect: [:m| m match]


You can also perform global search and string replacements, where the answer is a string with unmatched text left alone, and matched text replaced by the result of a call to a Block passed the ReMatch object as a single parameter. For example,

	('\w+' reGsearch: 'this is a test' sub: [:m| '<', (m match), '>']
and
(RePattern on: '\w+') gsearch: 'this is a test' sub: [:m| '<', (m match), '>']


return a string with each nonblank word surrounded by angle brackets.

To Search a String or Substring For Pattern Matches (Once Only):

As a convenience for those times when the compiled pattern object will be used for a single match and it is not necessary to retain the compiled pattern object, ReMatch provides functions to create ReMatch objects directly from a pattern and subject string:


'^Squ(ea|aw)k!$' reSearch: 'Squeak or Squawk!'
'^Squ(ea|aw)k!$' reSearch: 'Squeak or Squawk!' opt: 'imsxABEXZ'

ReMatch
on: 'Squ(ea|aw)k!$'
search: 'Squeak or Squawk!'
ReMatch
on: 'Squ(ea|aw)k'
search: 'Squeak or Squawk!'
opt: 'imsxABEXZ'
ReMatch
on: '^Squ(ea|aw)k!$'
search: 'Squeak or Squawk!'
from: 11
to: 17
ReMatch
on: 'Squ(ea|aw)k!'
search: 'Squeak or Squawk!'
from: 11
to: 17
opt: 'imsxABEXZ'
ReMatch on: aStr search: bStr from: 1
ReMatch on: aStr search: bStr from: 1 opt: optStr


Each of these expressions returns nil if no match is found in the string, or a corresponding ReMatch object if a match is found. An instance of ReMatch (call it "m") is generally created as a result of one of the above expressions, or as a result of a search of a string (call it "searchString") against a compiled regular expression RePattern (call it "re"). If you are making repeated searches, it is better first to generate a compiled expression object, and to perform searches using that object than to make repeated calls to these expressions.

Further information about the strings can be derived from the match expression, as follows:

Using ReMatch to Obtain Principal Match Information

The substring of searchString matched by re is given by:

	m match


which can be derived from searchString as follows:


m searchString
copyFrom: (m from)
to: (m to)


Using ReMatch to Obtain Captured Groups (and Collections of Captured Groups)

The number of substrings capturable by a parenthetical grouping in an re (regardless of the number actually matched to create m) is given by:

	m numGroups


The string captured by parenthetical grouping i, where 1=i=(m numGroups) is given by

	m matchAt: i


and this can be generated as follows:


m searchString
copyFrom: (m fromAt: i)
to: (m toAt: i)


And an array of size (m numGroups) can be generated from strings and indices accordingly:


m matches
m froms
m tos


II. Regular Expression Details



The syntax and semantics of the regular expressions supported by PCRE are
described below. Regular expressions are also described in the Perl
documentation and in a number of other books, some of which have copious
examples. Jeffrey Friedl's "Mastering Regular Expressions", published by
O'Reilly (ISBN 1-56592-257-3), covers them in great detail. The description
here is intended as reference documentation.

A regular expression is a pattern that is matched against a subject string from
left to right. Most characters stand for themselves in a pattern, and match the
corresponding characters in the subject. As a trivial example, the pattern

The quick brown fox


matches a portion of a subject string that is identical to itself. The power of
regular expressions comes from the ability to include alternatives and
repetitions in the pattern. These are encoded in the pattern by the use of
\fImeta-characters\fR, which do not stand for themselves but instead are
interpreted in some special way.

There are two different sets of meta-characters: those that are recognized
anywhere in the pattern except within square brackets, and those that are
recognized in square brackets. Outside square brackets, the meta-characters are
as follows:


\ general escape character with several uses
^ assert start of subject (or line, in multiline mode)
$ assert end of subject (or line, in multiline mode)
. match any character except newline (by default)
[ start character class definition
| start of alternative branch
( start subpattern
) end subpattern
? extends the meaning of (
also 0 or 1 quantifier
also quantifier minimizer
0 or more quantifier
+ 1 or more quantifier
{ start min/max quantifier
Part of a pattern that is in square brackets is called a "character class". In
a character class the only meta-characters are:

PRE
\ general escape character
^ negate the class, but only if the first character
- indicates character range
] terminates the character class
/PRE

The following sections describe the use of each of the meta-characters.

BBACKSLASH/B

The backslash character has several uses. Firstly, if it is followed by a
non-alphameric character, it takes away any special meaning that character may
have. This use of backslash as an escape character applies both inside and
outside character classes.

For example, if you want to match a "
" character, you write "\: 
" in the
pattern. This applies whether or not the following character would otherwise be
interpreted as a meta-character, so it is always safe to precede a
non-alphameric with "\" to specify that it stands for itself. In particular,
if you want to match a backslash, you write "\\".

If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
pattern (other than in a character class) and characters between a "#" outside
a character class and the next newline character are ignored. An escaping
backslash can be used to include a whitespace or "#" character as part of the
pattern.

A second use of backslash provides a way of encoding non-printing characters
in patterns in a visible manner. There is no restriction on the appearance of
non-printing characters, apart from the binary zero that terminates a pattern,
but when a pattern is being prepared by text editing, it is usually easier to
use one of the following escape sequences than the binary character it
represents:

\a     alarm, that is, the BEL character (hex 07)
\cx "control-x", where x is any character
\e escape (hex 1B)
\f formfeed (hex 0C)
\n newline (hex 0A)
\r carriage return (hex 0D)
\t tab (hex 09)
\xhh character with hex code hh
\ddd character with octal code ddd, or backreference
/PRE

The precise effect of "\cx" is as follows: if "x" is a lower case letter, it
is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
Thus "\cz" becomes hex 1A, but "\c{" becomes hex 3B, while "\c;" becomes hex
7B.

After "\x", up to two hexadecimal digits are read (letters can be in upper or
lower case).

After "\0" up to two further octal digits are read. In both cases, if there
are fewer than two digits, just those that are present are used. Thus the
sequence "\0\x\07" specifies two binary zeros followed by a BEL character.
Make sure you supply two digits after the initial zero if the character that
follows is itself an octal digit.

The handling of a backslash followed by a digit other than 0 is complicated.
Outside a character class, PCRE reads it and any following digits as a decimal
number. If the number is less than 10, or if there have been at least that many
previous capturing left parentheses in the expression, the entire sequence is
taken as a back reference. A description of how this works is given
later, following the discussion of parenthesized subpatterns.

Inside a character class, or if the decimal number is greater than 9 and there
have not been that many capturing subpatterns, PCRE re-reads up to three octal
digits following the backslash, and generates a single byte from the least
significant 8 bits of the value. Any subsequent digits stand for themselves.
For example:

PRE
\040 is another way of writing a space
\40 is the same, provided there are fewer than 40
previous capturing subpatterns
\7 is always a back reference
\11 might be a back reference, or another way of
writing a tab
\011 is always a tab
\0113 is a tab followed by the character "3"
\113 is the character with octal code 113 (since there
can be no more than 99 back references)
\377 is a byte consisting entirely of 1 bits
\81 is either a back reference, or a binary zero
followed by the two characters "8" and "1"
/PRE

Note that octal values of 100 or greater must not be introduced by a leading
zero, because no more than three octal digits are ever read.

All the sequences that define a single byte value can be used both inside and
outside character classes. In addition, inside a character class, the sequence
"\b" is interpreted as the backspace character (hex 08). Outside a character
class it has a different meaning (see below).

The third use of backslash is for specifying generic character types:

PRE
\d any decimal digit
\D any character that is not a decimal digit
\s any whitespace character
\S any character that is not a whitespace character
\w any "word" character
\W any "non-word" character
/PRE

Each pair of escape sequences partitions the complete set of characters into
two disjoint sets. Any given character matches one, and only one, of each pair.

A "word" character is any letter or digit or the underscore character, that is,
any character which can be part of a Perl "word". The definition of letters and
digits is controlled by PCRE's character tables, and may vary if locale-
specific matching is taking place (see "Locale support" above). For example, in
the "fr" (French) locale, some character codes greater than 128 are used for
accented letters, and these are matched by \w.

These character type sequences can appear both inside and outside character
classes. They each match one character of the appropriate type. If the current
matching point is at the end of the subject string, all of them fail, since
there is no character to match.

The fourth use of backslash is for certain simple assertions. An assertion
specifies a condition that has to be met at a particular point in a match,
without consuming any characters from the subject string. The use of
subpatterns for more complicated assertions is described below. The backslashed
assertions are

PRE
\b word boundary
\B not a word boundary
\A start of subject (independent of multiline mode)
\Z end of subject or newline at end (independent of multiline mode)
\z end of subject (independent of multiline mode)
/PRE

These assertions may not appear in character classes (but note that "\b" has a
different meaning, namely the backspace character, inside a character class).

A word boundary is a position in the subject string where the current character
and the previous character do not both match \w or \W (i.e. one matches
\w and the other matches \W), or the start or end of the string if the
first or last character matches \w, respectively.

The \A, \Z, and \z assertions differ from the traditional circumflex and
dollar (described below) in that they only ever match at the very start and end
of the subject string, whatever options are set. They are not affected by the
PCRE_NOTBOL or PCRE_NOTEOL options. The difference between \Z and \z is that
\Z matches before a newline that is the last character of the string as well
as at the end of the string, whereas \z matches only at the end.

BCIRCUMFLEX AND DOLLAR/B

Outside a character class, in the default matching mode, the circumflex
character is an assertion which is true only if the current matching point is
at the start of the subject string. Inside a character class, circumflex has an
entirely different meaning (see below).

Circumflex need not be the first character of the pattern if a number of
alternatives are involved, but it should be the first thing in each alternative
in which it appears if the pattern is ever to match that branch. If all
possible alternatives start with a circumflex, that is, if the pattern is
constrained to match only at the start of the subject, it is said to be an
"anchored" pattern. (There are also other constructs that can cause a pattern
to be anchored.)

A dollar character is an assertion which is true only if the current matching
point is at the end of the subject string, or immediately before a newline
character that is the last character in the string (by default). Dollar need
not be the last character of the pattern if a number of alternatives are
involved, but it should be the last item in any branch in which it appears.
Dollar has no special meaning in a character class.

The meaning of dollar can be changed so that it matches only at the very end of
the string, by setting the PCRE_DOLLAR_ENDONLY option at compile or matching
time. This does not affect the \Z assertion.

The meanings of the circumflex and dollar characters are changed if the
PCRE_MULTILINE option is set. When this is the case, they match immediately
after and immediately before an internal "\n" character, respectively, in
addition to matching at the start and end of the subject string. For example,
the pattern /^abc$/ matches the subject string "def\nabc" in multiline mode,
but not otherwise. Consequently, patterns that are anchored in single line mode
because all branches start with "^" are not anchored in multiline mode. The
PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.

Note that the sequences \A, \Z, and \z can be used to match the start and
end of the subject in both modes, and if all branches of a pattern start with
\A is it always anchored, whether PCRE_MULTILINE is set or not.

BFULL STOP (PERIOD, DOT)/B

Outside a character class, a dot in the pattern matches any one character in
the subject, including a non-printing character, but not (by default) newline.
If the PCRE_DOTALL option is set, then dots match newlines as well. The
handling of dot is entirely independent of the handling of circumflex and
dollar, the only relationship being that they both involve newline characters.
Dot has no special meaning in a character class.

BSQUARE BRACKETS/B

An opening square bracket introduces a character class, terminated by a closing
square bracket. A closing square bracket on its own is not special. If a
closing square bracket is required as a member of the class, it should be the
first data character in the class (after an initial circumflex, if present) or
escaped with a backslash.

A character class matches a single character in the subject; the character must
be in the set of characters defined by the class, unless the first character in
the class is a circumflex, in which case the subject character must not be in
the set defined by the class. If a circumflex is actually required as a member
of the class, ensure it is not the first character, or escape it with a
backslash.

For example, the character class [aeiou] matches any lower case vowel, while
[^aeiou] matches any character that is not a lower case vowel. Note that a
circumflex is just a convenient notation for specifying the characters which
are in the class by enumerating those that are not. It is not an assertion: it
still consumes a character from the subject string, and fails if the current
pointer is at the end of the string.

When caseless matching is set, any letters in a class represent both their
upper case and lower case versions, so for example, a caseless [aeiou] matches
"A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
caseful version would.

The newline character is never treated in any special way in character classes,
whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class
such as [^a] will always match a newline.

The minus (hyphen) character can be used to specify a range of characters in a
character class. For example, [d-m] matches any letter between d and m,
inclusive. If a minus character is required in a class, it must be escaped with
a backslash or appear in a position where it cannot be interpreted as
indicating a range, typically as the first or last character in the class.

It is not possible to have the literal character "]" as the end character of a
range. A pattern such as [W-]46] is interpreted as a class of two characters
("W" and "-") followed by a literal string "46]", so it would match "W46]" or
"-46]". However, if the "]" is escaped with a backslash it is interpreted as
the end of range, so [W-\]46] is interpreted as a single class containing a
range followed by two separate characters. The octal or hexadecimal
representation of "]" can also be used to end a range.

Ranges operate in ASCII collating sequence. They can also be used for
characters specified numerically, for example [\000-\037]. If a range that
includes letters is used when caseless matching is set, it matches the letters
in either case. For example, [W-c] is equivalent to [][\^_`wxyzabc], matched
caselessly, and if character tables for the "fr" locale are in use,
[\xc8-\xcb] matches accented E characters in both cases.

The character types \d, \D, \s, \S, \w, and \W may also appear in a
character class, and add the characters that they match to the class. For
example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
conveniently be used with the upper case character types to specify a more
restricted set of characters than the matching lower case type. For example,
the class [^\W_] matches any letter or digit, but not underscore.

All non-alphameric characters other than \, -, ^ (at the start) and the
terminating ] are non-special in character classes, but it does no harm if they
are escaped.

BVERTICAL BAR/B

Vertical bar characters are used to separate alternative patterns. For example,
the pattern

PRE gilbert|sullivan/PRE

matches either "gilbert" or "sullivan". Any number of alternatives may appear,
and an empty alternative is permitted (matching the empty string).
The matching process tries each alternative in turn, from left to right,
and the first one that succeeds is used. If the alternatives are within a
subpattern (defined below), "succeeds" means matching the rest of the main
pattern as well as the alternative in the subpattern.

BINTERNAL OPTION SETTING/B

The settings of PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and PCRE_EXTENDED
can be changed from within the pattern by a sequence of Perl option letters
enclosed between "(?" and ")". The option letters are

PRE
i for PCRE_CASELESS
m for PCRE_MULTILINE
s for PCRE_DOTALL
x for PCRE_EXTENDED
/PRE

For example, (?im) sets caseless, multiline matching. It is also possible to
unset these options by preceding the letter with a hyphen, and a combined
setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
permitted. If a letter appears both before and after the hyphen, the option is
unset.

The scope of these option changes depends on where in the pattern the setting
occurs. For settings that are outside any subpattern (defined below), the
effect is the same as if the options were set or unset at the start of
matching. The following patterns all behave in exactly the same way:

PRE
(?i)abc
a(?i)bc
ab(?i)c
abc(?i)
/PRE

which in turn is the same as compiling the pattern abc with PCRE_CASELESS set.
In other words, such "top level" settings apply to the whole pattern (unless
there are other changes inside subpatterns). If there is more than one setting
of the same option at top level, the rightmost setting is used.

If an option change occurs inside a subpattern, the effect is different. This
is a change of behaviour in Perl 5.005. An option change inside a subpattern
affects only that part of the subpattern that follows it, so

PRE (a(?i)b)c/PRE

matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
By this means, options can be made to have different settings in different
parts of the pattern. Any changes made in one alternative do carry on
into subsequent branches within the same subpattern. For example,

PRE (a(?i)b|c)/PRE

matches "ab", "aB", "c", and "C", even though when matching "C" the first
branch is abandoned before the option setting. This is because the effects of
option settings happen at compile time. There would be some very weird
behaviour otherwise.

The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the
same way as the Perl-compatible options by using the characters U and X
respectively. The (?X) flag setting is special in that it must always occur
earlier in the pattern than any of the additional features it turns on, even
when it is at top level. It is best put at the start.

BSUBPATTERNS/B

Subpatterns are delimited by parentheses (round brackets), which can be nested.
Marking part of a pattern as a subpattern does two things:

1. It localizes a set of alternatives. For example, the pattern

PRE cat(aract|erpillar|)/PRE

matches one of the words "cat", "cataract", or "caterpillar". Without the
parentheses, it would match "cataract", "erpillar" or the empty string.

2. It sets up the subpattern as a capturing subpattern (as defined above).
When the whole pattern matches, that portion of the subject string that matched
the subpattern is passed back to the caller via the \fIovector\fR argument of
pcre_exec(). Opening parentheses are counted from left to right (starting
from 1) to obtain the numbers of the capturing subpatterns.

For example, if the string "the red king" is matched against the pattern

PRE the ((red|white) (king|queen))/PRE

the captured substrings are "red king", "red", and "king", and are numbered 1,
2, and 3.

The fact that plain parentheses fulfil two functions is not always helpful.
There are often times when a grouping subpattern is required without a
capturing requirement. If an opening parenthesis is followed by "?:", the
subpattern does not do any capturing, and is not counted when computing the
number of any subsequent capturing subpatterns. For example, if the string "the
white queen" is matched against the pattern

PRE the ((?:red|white) (king|queen))/PRE

the captured substrings are "white queen" and "queen", and are numbered 1 and
2. The maximum number of captured substrings is 99, and the maximum number of
all subpatterns, both capturing and non-capturing, is 200.

As a convenient shorthand, if any option settings are required at the start of
a non-capturing subpattern, the option letters may appear between the "?" and
the ":". Thus the two patterns

PRE
(?i:saturday|sunday)
(?:(?i)saturday|sunday)
/PRE

match exactly the same set of strings. Because alternative branches are tried
from left to right, and options are not reset until the end of the subpattern
is reached, an option setting in one branch does affect subsequent branches, so
the above patterns match "SUNDAY" as well as "Saturday".

BREPETITION/B

Repetition is specified by quantifiers, which can follow any of the following
items:

PRE
a single character, possibly escaped
the . metacharacter
a character class
a back reference (see next section)
a parenthesized subpattern (unless it is an assertion - see below)
/PRE

The general repetition quantifier specifies a minimum and maximum number of
permitted matches, by giving the two numbers in curly brackets (braces),
separated by a comma. The numbers must be less than 65536, and the first must
be less than or equal to the second. For example:

PRE z{2,4}/PRE

matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
character. If the second number is omitted, but the comma is present, there is
no upper limit; if the second number and the comma are both omitted, the
quantifier specifies an exact number of required matches. Thus

PRE [aeiou]{3,}/PRE

matches at least 3 successive vowels, but may match many more, while

PRE \d{8}/PRE

matches exactly 8 digits. An opening curly bracket that appears in a position
where a quantifier is not allowed, or one that does not match the syntax of a
quantifier, is taken as a literal character. For example, {,6} is not a
quantifier, but a literal string of four characters.

The quantifier {0} is permitted, causing the expression to behave as if the
previous item and the quantifier were not present.

For convenience (and historical compatibility) the three most common
quantifiers have single-character abbreviations:

PRE
* is equivalent to {0,}
+ is equivalent to {1,}
? is equivalent to {0,1}
/PRE

It is possible to construct infinite loops by following a subpattern that can
match no characters with a quantifier that has no upper limit, for example:

PRE (a?)*/PRE

Earlier versions of Perl and PCRE used to give an error at compile time for
such patterns. However, because there are cases where this can be useful, such
patterns are now accepted, but if any repetition of the subpattern does in fact
match no characters, the loop is forcibly broken.

By default, the quantifiers are "greedy", that is, they match as much as
possible (up to the maximum number of permitted times), without causing the
rest of the pattern to fail. The classic example of where this gives problems
is in trying to match comments in C programs. These appear between the
sequences /* and */ and within the sequence, individual &star and / characters may
appear. An attempt to match C comments by applying the pattern

PRE /\*.*\*//PRE

to the string

PRE /* first command */ not comment /* second comment *//PRE

fails, because it matches the entire string due to the greediness of the .
item.

However, if a quantifier is followed by a question mark, then it ceases to be
greedy, and instead matches the minimum number of times possible, so the
pattern

  /\*.?\*/


does the right thing with the C comments. The meaning of the various
quantifiers is not otherwise changed, just the preferred number of matches.
Do not confuse this use of question mark with its use as a quantifier in its
own right. Because it has two uses, it can sometimes appear doubled, as in

  \d??\d


which matches one digit by preference, but can match two if that is the only
way the rest of the pattern matches.

If the PCRE_UNGREEDY option is set (an option which is not available in Perl)
then the quantifiers are not greedy by default, but individual ones can be made
greedy by following them with a question mark. In other words, it inverts the
default behaviour.

When a parenthesized subpattern is quantified with a minimum repeat count that
is greater than 1 or with a limited maximum, more store is required for the
compiled pattern, in proportion to the size of the minimum or maximum.

If a pattern starts with . then it is implicitly anchored, since whatever
follows will be tried against every character position in the subject string.
PCRE treats this as though it were preceded by \A.

When a capturing subpattern is repeated, the value captured is the substring
that matched the final iteration. For example, after

(tweedle[dume]{3}\s*)+/PRE

has matched "tweedledum tweedledee" the value of the captured substring is
"tweedledee". However, if there are nested capturing subpatterns, the
corresponding captured values may have been set in previous iterations. For
example, after

PRE /(a|(b))+//PRE

matches "aba" the value of the second captured substring is "b".

BBACK REFERENCES/B

Outside a character class, a backslash followed by a digit greater than 0 (and
possibly further digits) is a back reference to a capturing subpattern earlier
(i.e. to its left) in the pattern, provided there have been that many previous
capturing left parentheses.

However, if the decimal number following the backslash is less than 10, it is
always taken as a back reference, and causes an error only if there are not
that many capturing left parentheses in the entire pattern. In other words, the
parentheses that are referenced need not be to the left of the reference for
numbers less than 10. See the section entitled "Backslash" above for further
details of the handling of digits following a backslash.


A back reference matches whatever actually matched the capturing subpattern in
the current subject string, rather than anything matching the subpattern
itself. So the pattern

PRE (sens|respons)e and \1ibility/PRE

matches "sense and sensibility" and "response and responsibility", but not
"sense and responsibility". If caseful matching is in force at the time of the
back reference, then the case of letters is relevant. For example,

PRE ((?i)rah)\s+\1/PRE

matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
capturing subpattern is matched caselessly.

There may be more than one back reference to the same subpattern. If a
subpattern has not actually been used in a particular match, then any back
references to it always fail. For example, the pattern

PRE (a|(bc))\2/PRE

always fails if it starts to match "a" rather than "bc". Because there may be
up to 99 back references, all digits following the backslash are taken
as part of a potential back reference number. If the pattern continues with a
digit character, then some delimiter must be used to terminate the back
reference. If the PCRE_EXTENDED option is set, this can be whitespace.
Otherwise an empty comment can be used.

A back reference that occurs inside the parentheses to which it refers fails
when the subpattern is first used, so, for example, (a\1) never matches.
However, such references can be useful inside repeated subpatterns. For
example, the pattern

PRE (a|b\1)+/PRE

matches any number of "a"s and also "aba", "ababaa" etc. At each iteration of
the subpattern, the back reference matches the character string corresponding
to the previous iteration. In order for this to work, the pattern must be such
that the first iteration does not need to match the back reference. This can be
done using alternation, as in the example above, or by a quantifier with a
minimum of zero.


BASSERTIONS/B

An assertion is a test on the characters following or preceding the current
matching point that does not actually consume any characters. The simple
assertions coded as \b, \B, \A, \Z, \z, ^ and $ are described above. More
complicated assertions are coded as subpatterns. There are two kinds: those
that look ahead of the current position in the subject string, and those that
look behind it.

An assertion subpattern is matched in the normal way, except that it does not
cause the current matching position to be changed. Lookahead assertions start
with (?= for positive assertions and (?! for negative assertions. For example,

PRE \w+(?=;)/PRE

matches a word followed by a semicolon, but does not include the semicolon in
the match, and

PRE foo(?!bar)/PRE

matches any occurrence of "foo" that is not followed by "bar". Note that the
apparently similar pattern

PRE (?!foo)bar/PRE

does not find an occurrence of "bar" that is preceded by something other than
"foo"; it finds any occurrence of "bar" whatsoever, because the assertion
(?!foo) is always true when the next three characters are "bar". A
lookbehind assertion is needed to achieve this effect.

Lookbehind assertions start with (?= for positive assertions and (?! for
negative assertions. For example,

PRE (?!foo)bar/PRE

does find an occurrence of "bar" that is not preceded by "foo". The contents of
a lookbehind assertion are restricted such that all the strings it matches must
have a fixed length. However, if there are several alternatives, they do not
all have to have the same fixed length. Thus

PRE (?=bullock|donkey)/PRE

is permitted, but

PRE (?!dogs?|cats?)/PRE

causes an error at compile time. Branches that match different length strings
are permitted only at the top level of a lookbehind assertion. This is an
extension compared with Perl 5.005, which requires all branches to match the
same length of string. An assertion such as

PRE (?=ab(c|de))/PRE

is not permitted, because its single top-level branch can match two different
lengths, but it is acceptable if rewritten to use two top-level branches:

PRE (?=abc|abde)/PRE

The implementation of lookbehind assertions is, for each alternative, to
temporarily move the current position back by the fixed width and then try to
match. If there are insufficient characters before the current position, the
match is deemed to fail. Lookbehinds in conjunction with once-only subpatterns
can be particularly useful for matching at the ends of strings; an example is
given at the end of the section on once-only subpatterns.

Several assertions (of any sort) may occur in succession. For example,

PRE (?=\d{3})(?!999)foo/PRE

matches "foo" preceded by three digits that are not "999". Furthermore,
assertions can be nested in any combination. For example,

PRE (?=(?!foo)bar)baz/PRE

matches an occurrence of "baz" that is preceded by "bar" which in turn is not
preceded by "foo".

Assertion subpatterns are not capturing subpatterns, and may not be repeated,
because it makes no sense to assert the same thing several times. If an
assertion contains capturing subpatterns within it, these are always counted
for the purposes of numbering the capturing subpatterns in the whole pattern.
Substring capturing is carried out for positive assertions, but it does not
make sense for negative assertions.

Assertions count towards the maximum of 200 parenthesized subpatterns.

BONCE-ONLY SUBPATTERNS/B

With both maximizing and minimizing repetition, failure of what follows
normally causes the repeated item to be re-evaluated to see if a different
number of repeats allows the rest of the pattern to match. Sometimes it is
useful to prevent this, either to change the nature of the match, or to cause
it fail earlier than it otherwise might, when the author of the pattern knows
there is no point in carrying on.

Consider, for example, the pattern \d+foo when applied to the subject line

PRE 123456bar/PRE

After matching all 6 digits and then failing to match "foo", the normal
action of the matcher is to try again with only 5 digits matching the \d+
item, and then with 4, and so on, before ultimately failing. Once-only
subpatterns provide the means for specifying that once a portion of the pattern
has matched, it is not to be re-evaluated in this way, so the matcher would
give up immediately on failing to match "foo" the first time. The notation is
another kind of special parenthesis, starting with (? as in this example:

PRE (?\d+)bar/PRE

This kind of parenthesis "locks up" the part of the pattern it contains once
it has matched, and a failure further into the pattern is prevented from
backtracking into it. Backtracking past it to previous items, however, works as
normal.

An alternative description is that a subpattern of this type matches the string
of characters that an identical standalone pattern would match, if anchored at
the current point in the subject string.

Once-only subpatterns are not capturing subpatterns. Simple cases such as the
above example can be thought of as a maximizing repeat that must swallow
everything it can. So, while both \d+ and \d+? are prepared to adjust the
number of digits they match in order to make the rest of the pattern match,
(?\d+) can only match an entire sequence of digits.

This construction can of course contain arbitrarily complicated subpatterns,
and it can be nested.

Once-only subpatterns can be used in conjunction with lookbehind assertions to
specify efficient matching at the end of the subject string. Consider a simple
pattern such as

PRE abcd$/PRE

when applied to a long string which does not match it. Because matching
proceeds from left to right, PCRE will look for each "a" in the subject and
then see if what follows matches the rest of the pattern. If the pattern is
specified as

PRE .abcd$


then the initial . matches the entire string at first, but when this fails, it
backtracks to match all but the last character, then all but the last two
characters, and so on. Once again the search for "a" covers the entire string,
from right to left, so we are no better off. However, if the pattern is written
as

(?.*)(?=abcd)/PRE

then there can be no backtracking for the . item; it can match only the entire
string. The subsequent lookbehind assertion does a single test on the last four
characters. If it fails, the match fails immediately. For long strings, this
approach makes a significant difference to the processing time.

CONDITIONAL SUBPATTERNS

It is possible to cause the matching process to obey a subpattern
conditionally or to choose between two alternative subpatterns, depending on
the result of an assertion, or whether a previous capturing subpattern matched
or not. The two possible forms of conditional subpattern are


(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)


If the condition is satisfied, the yes-pattern is used; otherwise the
no-pattern (if present) is used. If there are more than two alternatives in the
subpattern, a compile-time error occurs.

There are two kinds of condition. If the text between the parentheses consists
of a sequence of digits, then the condition is satisfied if the capturing
subpattern of that number has previously matched. Consider the following
pattern, which contains non-significant white space to make it more readable
(assume the PCRE_EXTENDED option) and to divide it into three parts for ease
of discussion:

  ( \( )?    [^()]+    (?(1) \) )


The first part matches an optional opening parenthesis, and if that
character is present, sets it as the first captured substring. The second part
matches one or more characters that are not parentheses. The third part is a
conditional subpattern that tests whether the first set of parentheses matched
or not. If they did, that is, if subject started with an opening parenthesis,
the condition is true, and so the yes-pattern is executed and a closing
parenthesis is required. Otherwise, since no-pattern is not present, the
subpattern matches nothing. In other words, this pattern matches a sequence of
non-parentheses, optionally enclosed in parentheses.

If the condition is not a sequence of digits, it must be an assertion. This may
be a positive or negative lookahead or lookbehind assertion. Consider this
pattern, again containing non-significant white space, and with the two
alternatives on the second line:


(?(?=[^a-z]*[a-z])
\d{2}[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )


The condition is a positive lookahead assertion that matches an optional
sequence of non-letters followed by a letter. In other words, it tests for the
presence of at least one letter in the subject. If a letter is found, the
subject is matched against the first alternative; otherwise it is matched
against the second. This pattern matches strings in one of the two forms
dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.


COMMENTS

The sequence (?# marks the start of a comment which continues up to the next
closing parenthesis. Nested parentheses are not permitted. The characters
that make up a comment play no part in the pattern matching at all.

If the PCRE_EXTENDED option is set, an unescaped # character outside a
character class introduces a comment that continues up to the next newline
character in the pattern.


III. Performance



Certain items that may appear in patterns are more efficient than others. It is
more efficient to use a character class like [aeiou] than a set of alternatives
such as (a|e|i|o|u). In general, the simplest construction that provides the
required behaviour is usually the most efficient. Jeffrey Friedl's book
contains a lot of discussion about optimizing regular expressions for efficient
performance.

IV. Limitations



There are some size limitations in PCRE but it is hoped that they will never in
practice be relevant.

The maximum length of a compiled pattern is 65539 (sic) bytes.
All values in repeating quantifiers must be less than 65536.
The maximum number of capturing subpatterns is 99.
The maximum number of all parenthesized subpatterns, including capturing subpatterns, assertions, and other types of subpattern, is 200.

The maximum length of a subject string is the largest positive number that an
integer variable can hold. However, PCRE uses recursion to handle subpatterns
and indefinite repetition. This means that the available stack space may limit
the size of a subject string that can be processed by certain patterns.

V. Differences From Perl



The differences described here are with respect to Perl 5.005.

1. By default, a whitespace character is any character that the C library
function isspace() recognizes, though it is possible to compile PCRE with
alternative character type tables. Normally \fBisspace()\fR matches space,
formfeed, newline, carriage return, horizontal tab, and vertical tab. Perl 5
no longer includes vertical tab in its set of whitespace characters. The \v
escape that was in the Perl documentation for a long time was never in fact
recognized. However, the character itself was treated as whitespace at least
up to 5.002. In 5.004 and 5.005 it does not match \s.

2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl permits
them, but they do not mean what you might think. For example, (?!a){3} does
not assert that the next three characters are not "a". It just asserts that the
next character is not "a" three times.

3. Capturing subpatterns that occur inside negative lookahead assertions are
counted, but their entries in the offsets vector are never set. Perl sets its
numerical variables from any such patterns that are matched before the
assertion fails to match something (thereby succeeding), but only if the
negative lookahead assertion contains just one branch.

4. Though binary zero characters are supported in the subject string, they are
not allowed in a pattern string because it is passed as a normal C string,
terminated by zero. The escape sequence "\0" can be used in the pattern to
represent a binary zero.

5. The following Perl escape sequences are not supported: \l, \u, \L, \U,
\E, \Q. In fact these are implemented by Perl's general string-handling and
are not part of its pattern matching engine.

6. The Perl \G assertion is not supported as it is not relevant to single
pattern matches.

7. Fairly obviously, PCRE does not support the (?{code}) construction.

8. There are at the time of writing some oddities in Perl 5.005_02 concerned
with the settings of captured strings when part of a pattern is repeated. For
example, matching "aba" against the pattern /^(a(b)?)+$/ sets $2 to the value
"b", but matching "aabbaa" against /^(aa(bb)?)+$/ leaves $2 unset. However, if
the pattern is changed to /^(aa(b(b))?)+$/ then $2 (and $3) get set.

In Perl 5.004 $2 is set in both cases, and that is also true of PCRE. If in the
future Perl changes to a consistent state that is different, PCRE may change to
follow.

9. Another as yet unresolved discrepancy is that in Perl 5.005_02 the pattern
/^(a)?(?(1)a|b)+$/ matches the string "a", whereas in PCRE it does not.
However, in both Perl and PCRE /^(a)?a/ matched against "a" leaves $1 unset.

10. PCRE provides some extensions to the Perl regular expression facilities:

(a) Although lookbehind assertions must match fixed length strings, each
alternative branch of a lookbehind assertion can match a different length of
string. Perl 5.005 requires them all to have the same length.

(b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ meta-
character matches only at the very end of the string.

(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
meaning is faulted.

(d) If PCRE_UNGREEDY is set, the greediness of the repetition quantifiers is
inverted, that is, by default they are not greedy, but if followed by a
question mark they are.


Andrew C. Greenberg

Fix Documentation