develooper Front page | perl.perl6.announce.rfc | Postings from October 2000

RFC 332 (v2) Regex: Make /$/ equivalent to /\z/ under the '/s' modifier

From:
Perl6 RFC Librarian
Date:
October 1, 2000 17:51
Subject:
RFC 332 (v2) Regex: Make /$/ equivalent to /\z/ under the '/s' modifier
Message ID:
20001002004617.1977.qmail@tmtowtdi.perl.org
This and other RFCs are available on the web at
  http://dev.perl.org/rfc/

=head1 TITLE

Regex: Make /$/ equivalent to /\z/ under the '/s' modifier

=head1 VERSION

  Maintainer: Bart Lateur <bart.lateur@skynet.be>
  Date: 28 Sep 2000
  Last Modified: 1 Oct 2000
  Mailing List: perl6-language-regex@perl.org
  Number: 332
  Version: 2
  Status: Frozen

=head1 ABSTRACT

To most Perlers, /$/ in a regex simply means "end of string". This is
only right, if you're absolutely sure your string doesn't end in a
newline, as is commonly the case in a large part of all textual data:
ordinary strings don't contain newlines. Lines coming from text files
can generally only contain a newline as the very last character. The
'/s' modifier is usually only used in combination with the former class
of textual data.

However, this situation is basically a bug hole.

This RFC proposes to change the '/s' modifier so that under '/s', /$/
will only match at the very end of a string, and not also before a
newline at the end of the string.

=head1 CHANGES

=over 4

=item *
Added section about alternatives, such as the addition of the /$$/
syntax

=item *
Added section about /z/ and /\Z/

=item *
Expanded section on '/ms'.

=item *
Complete rewrite of "MIGRATION" section

=back

=head1 NOTES ON FREEZING

Since nobody had any more remarks on the draft, and the deadline is
nigh, I'm freezing it as is.

=head1 DESCRIPTION

To most Perl programmers, /^foo$/ is a regex that can only match the
string "foo". It's not, actually: it can match "foo\n", too. This
assumption is usually safe, because people know the kind of data they're
dealing with, and they "know" that it won't ever end in a newline.

However, this basically is a chance for bugs to creep in, if for some
reason this assumption about the data no longer holds.

To make matters worse, Perl doesn't even have a mechanism to prevent the
regex engine from matching /$/ at just before the last character if it's
a newline.

Originally, we had thought of adding Yet Another Regex Modifier; but to
be honest, having 2 modifiers just for the newline is already confusing
enough, for too many people. A third is definitely out.

Therefore, the proposal is instead to modify the behaviour of the '/s'
modifier.

Under '/s':

=over 2

=item *

/./ can match any character, including newline;

=item *

/$/ can match only at the very end of the string, not also in front of a
last character, if it happens to be a newline.

=back

This seems simple enough.

=head1 CONSIDERATIONS

=head2 Mnemonic value of '/s'

'/s' originally stood for "single line". This can no longer be true, the
mnemonic value of the "s" is thereby reduced to zero.

However, the mnemonic value wasn't that great to begin with, especially
if you consider that combining '/s' and '/m' is not only possible, but a
useful option, too. How can a string both be a single line and
multiline, at the same time?

So, to most Perl programmers, '/s' simply stands for

=over 2

=item
let /./ match a newline too

=back

which now gets turned into:

=over 2

=item
treat "\n" as an ordinary character

=back

The change isn't that big, so it is just as easy to remember. Or not.

=head2 The $* variable

'/s' and '/m' also have a lesser known side effect: they both override
the setting of the $* special variable, which controls multiline related
behaviour in regexes.

Use of this special variable has already been deprecated at least since
Perl5 first came out, more than 5 years ago. It is a very good candidate
to be removed from Perl6 altogether, which would result in fewer
gotcha's in the language. That is a Good Thing.

Perlvar says:

    Use of `$*' is deprecated in modern Perl, supplanted by the `/s'
    and `/m' modifiers on pattern matching.


Therefore, any changing behaviour of '/s', with regards to $*, can
nowadays hardly be considered relevant, any more.

See also L<RFC 347: Remove long-deprecated $*>

=head2 '/ms': combined '/m' and '/s'

The '/m' option makes /$/ match either at the end of the string, or
before I<any> newline. Adding the '/s' modifier won't change that. As a
result, '/ms' still works as before. Internally, '/m' has taken over the
job of matching before a newline at the very end of the string, simply
because /$/m can match before I<every> newline.

=head2 /\z/ and /\Z/

The behaviour of /\z/ and /<Z/ will remain unaltered, under all
cicumstances.

Currently, /$/ is a synonym for /\Z/, even under '/s'. With the modified
'/s', /\Z/ will retain its old meaning, thereby these will no longer be
synonyms under all circumstances.

=head1 MIGRATION

The Perl5 To Perl6 converter can replace every occurence of

    /$/s

with 

    /\Z/s


However, it's not unlikely that currently having /$/s in your regexes,
is actually a bug in your script, but you don't care because the data
won't ever make it visible. A warning if /$/ is found in combination
with a bare '/s' modifier, not combined with '/m', in addition to
replacing it with /\Z/, might be a nice idea.

=head1 IMPLEMENTATION

Under '/s', make '$' behave as /\z/ does now.

=head1 ALTERNATIVES

Some people, in particular Hugo, would rather add a special syntax for
this special case, and leave /$/ under '/s' alone. This seems not to win
the favour of too many other people (actually, the sample is too small
to get reliable statistics), for these reasons:

=over 2

=item *
C<$$> already has a meaning: it is the special variable containing the
PID.

=item *
We already I<have> an alternative: /\z/. We don't want to just add Yet
Another Alternative. We want /$/ to do the Right Thing.

=back

=head1 REFERENCES

perlre, about '/s' and '/m'

perlvar, section about $*

RFC 347: Remove long-deprecated $* (aka $MULTILINE_MATCHING)





nntp.perl.org: Perl Programming lists via nntp and http.
Comments to Ask Bjørn Hansen at ask@perl.org | Group listing | About