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Boolean Regexes


  Maintainer: Richard Proctor <richard@waveney.org>
  Date: 6 Sep 2000
  Last Modified: 1 Oct 2000
  Mailing List: perl6-language-regex@perl.org
  Number: 198
  Version: 3
  Status: Frozen


This is a development of the proposal for the "not a pattern" concept in RFC 166 V1. Looking deeper into the handling of advanced regexs, there are potential needs for many other concepts, to allow a regex to extract information directly from a complex file in one go, rather than a mixture of splits and nested regexes as is typically needed today. With these parsing data should become easier (in some cases).


V2 - Changed the "Fail Pattern", enhanced the wording for many things.

V3 - Further clarifications


It would be nice (in my opinion) to be able to build more elaborate regexes allowing data to be mined out of a sting in one go. These ideas allow you to apply several patterns to one substring (each must match), to fail a match from within, to look for patterns that do not contain other patterns, and to handle looking for cases such as (foo.*bar)|(bar.*foo) in a more general way of saying "A substring that contains both foo and bar".

These are ideas, at present with some proposed syntax. The ideas are more important than the exact syntax at this stage. This is very much work in progress.

I have called these boolean regexs as they bring the concepts of and (&&) or (||) and not(!) into the realm of regexes.

Within a boolean regex (or the boolean part of a regex), several new symbols have meanings, and some have enhanced meanings.

The Ideas

Are these part of a boolean (?&...) construct within an existing regex, or is the advanced syntax (and meaning of &|!^$) invoked by a new flag such as /B?

These can look like line noise so the use of white space with /x is used throughout, and it might be appropriate to enforce (or assume) /x within (&...).

I am not alone in wanting something like this, Ilya in a posting of lots of things he would like had :

   onion rings: (?<> A <> B &! C & D)  (substring matched by A
				        such that B and D match against
					it, but C does not, in B, C, D
					\A and \z denote boundaries of
				        what was matched by A);

Which is virtually identical in concept.

Boolean construct

(?&...) grabs a substring, and applies one or more tests to the substring.

Substring matching multiple patterns (&&)

(?& pattern1 && pattern2 && pattern3 )

A substring is definied that matches each pattern.

For example, the first pattern may say specify a substring of at least 30 chars, the next two have a foo and a bar.

Substring matching alternative patterns (||)

(?& pattern1 || pattern2 || pattern3)

This is similar to the existing alternative syntax "|" but the alternatives to "|" behave as /^pattern$/ rather than /pattern/ (^ and $ taken as refereing to the substring in this case - see below).

(pattern1 || pattern2 || pattern3) can be mixed in with the && case above to build up more advanced cases. && and || operators can be nested with brackets in normal ways.

Brackets within boolean regexes

Within a complex boolean regex there are likely to be lots and lots of brackets to nest and control the behaviour of the regex. Rather than having to sprinkle the regex with (?:) line noise, it would be nicer to just use ordinary brackets () and only support capturing of elements by using one of the (?$=) or (?%=) constructs that have been proposed elsewhere (RFC 112 and RFC 150). There might be some case for this as a general capability using some flag /b = brackets?

Substring not matching a pattern

In RFC 166 I originally proposed (?^ pattern ). This proposal replaces that. Though it could be used as well outside of the (?&) construct.

!pattern matches anything that does not match the pattern. On its own it is not terribly useful, but in conjuction with && and || one can do things such as /(?& <img && ! alt=)/ ie does it have an image not have an alt.

! is chosen as it has the same basic meaning outside of regexes.

!pattern is a non greedy construct that matches any string/substring that does not match the pattern.

Meaning of $ and ^ inside a boolean regex

^ and $ are taken to mean the begining and end of the substring, not begining and and of the line/string from within a boolean regex.


Should the (?&...) construct be greedy or nongreedy? To some extent this depends on the elements it contains. If all the matching set of patterns are greedy then it will be greedy, if they are not greedy then it will not be. This might or might be sufficient.

If the situation is ambiguous (or might be) The boolean can be expresed as (?&? ...) to force non greediness.

Delivering a substring to some code that generates a pass/fail

(?*{code}) delivers a substring to the code, which returns with success or failure. The code sees the substring as $_. This is not dependant on the Boolean regex concept and could be used for other things, though it is most useful in this context.

This is sort of equivalent to (?: (.*)(??{$_ = $1; code})) ie it matches an arbitary long substring and deliveres it to the code. But not dependant on how many brackets have been used already. If expressed this way the code needs to deliver succes and failure.

I am not sure how greedy the capture element should be. ie if it should be (.*) or (.*?) or (.+) or (.+?)] but I think (.*) is suficient given that it may be bound by other regexes in a boolean context to reduce the context sufficiently.

A Failure Token

\F if reached as part of a pattern is always a failure. This can be used outside of the (?& construct. This is not dependant on the Boolean regex concept and could be used for other things.

(In RFC 198 V1 this was proposed as (?F) but I think \F is more in keeping with existing and intended syntax.)

There might be a case for a compilentary Success Token \T ?? though I am not sure it is needed.

Applications and examples

Matching both foo and bar in a string.

	$string =~ /(?& foo && bar)/x;

Matching a string that does not contain baz that is at least 20 chars between foo and bar.

	$string =~ /foo (?& .{20,} && ! baz ) bar/x;

Does a html image have both an alt and a src, and what are they?

	$string =~ /<img(?& \s alt=(?$Alt=(".*?"|\S*)) &&
			    \s src=(?$Src=(".*?"|\S*)) )>/ix;

It might be possible to have a regex that simply matches valid perl6 out of this. (though it would be large...)!


Implementation detail is not appropriate for this stage in the devlopment of this RFC. If the concepts gain approval then detailed implementation issues become relevant.

There are two aspects to regexs - compiling and executing:

Compiling of these extended forms should be relativly straight forward, but would need some extensions to recognise the regex as being within (?&) state to handle the extended syntax.

Executing - No thoughts at all at present.


RFC 166: Alternative lists and quoting of things

RFC 112: Assignment within a regex

RFC 150: Extend regex syntax to provide for return of a hash of matched subpatterns

RFC 145: Brace-matching for Perl Regular Expressions (or at least the discussion that followed it)