Re: Common error with | and ^$ in regexps
Juerd
> Will the relative precedence of grouping versus anchors for beginning and
> end of line remain the same in Perl6 rules?
There currently is no such thing as precedence in regexes. Changing this
would make understanding regexes a lot harder, I think.
And now that (?:) is called [], I think teaching how to just do the
right thing is easy enough.
/^(?:foo|bar|baz)$/
/^ [ foo | bar | baz ] $/
Juerd
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Nicholas Clark
end of line remain the same in Perl6 rules?
The error of writing
/^(?:free|net|open)bsd|bsdos|interix$/
when you mean
/^(?:(?:free|net|open)bsd|bsdos|interix)$/
is rather too easy to make. This is not the first time I've seen this sort
of error, and I think I've made it myself at least once.
My gut feeling is that the need to write expressions that behave as:
/(?:^(?:free|net|open)bsd)|bsdos|(?:interix$)/
is actually very rare.
Nicholas Clark
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Date: Mon, 7 Feb 2005 05:35:00 -0500
From: Michael G Schwern <sch...@pobox.com>
To: Rafael Garcia-Suarez <rgarci...@mandrakesoft.com>
Cc: make...@perl.org
Subject: Re: Where were we at?
In-Reply-To: <slrncvk15b.1cp...@grubert.mandrakesoft.com>
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On Fri, Jan 28, 2005 at 09:23:55AM -0000, Rafael Garcia-Suarez wrote:
> Please don't forget to integrate this platform-specific patch from
> bleadperl :
>
> Change 23849 by rgs@grubert on 2005/01/21 15:26:10
>
> Subject: [perl #33892] Add Interix support
> From: Todd Vierling (via RT) <perlbug-...@perl.org>
> Date: 21 Jan 2005 14:36:31 -0000
> Message-ID: <rt-3.0.11-33892-106...@perl.org>
I think the patch is wrong, or adding more wrongness.
-$Is_BSD = $^O =~ /^(?:free|net|open)bsd|bsdos$/;
+$Is_BSD = $^O =~ /^(?:free|net|open)bsd|bsdos|interix$/;
That second pair of bsd|bsdos isn't enclosed in parens. Furthermore,
its not "freeinterix" its just "interix".
Here's the right line.
$Is_BSD = $^O =~ /^(?:free|net|open)bsd$/ or
$^O eq 'bsdos' or $^O eq 'interix';
----- End forwarded message -----
Patrick R. Michaud
> Nicholas Clark skribis 2005-02-07 12:10 (+0000):
> > Will the relative precedence of grouping versus anchors for beginning and
> > end of line remain the same in Perl6 rules?
>
> There currently is no such thing as precedence in regexes. Changing this
> would make understanding regexes a lot harder, I think.
A clarification:
P6rules currently has precedence. Alternatives is one
of the looser bindings. I suspect the original question is really
asking about the "relative precedence of [alternatives] versus
anchors for beginning and end of [string]", since the question arises
even in the absence of (?:...) constructs.
Here's a brief stab at a p6 rule expression "precedence table", at least
for what I've been working with in the grammar engine:
terms a . \s \b ^ $ ^^ $$ (...) [...] <...> :: :::
quantifiers * + ? *? +? ?? **{...} **{...}?
backtracking :
concatenation
conjunctive &
alternative |
Thus Nicholas' question is really asking if we can give ^ and $ a
looser binding than alternatives, such that
/ ^ abc | def | ghi $ /
binds as
/ ^ [ abc | def | ghi ] $/
and not
/ [^abc] | def | [ghi$] /
It's certainly technically possible to do this, but I'd then wonder what
to do about ^^ and $$, and if it would then be more confusing that
^ and $ (and possibly ^^ and $$) bind much more loosely than the
other assertions. Personally, I'll let you guys hash out those things
and then set the grammar engine to match. :-)
Pm