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Subject Author Date
Some questions about q{} and qr{}. Robbie Hatley 04-13-2008
Posted by Robbie Hatley on April 13, 2008, 9:07 pm
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Today I was editing a URL-likifying program I wrote several
weeks ago, and I ran across some issues with q{} and qr{}
which are puzzling me.

Here's an edited-for-brevity version of the program:

my $Legal = q;
my $Regex1 = qr
my $Regex2 = qr;
while (<>)
{
sg;
s{\n<p><a href="$1">$1</a></p>\n}g;
print ($_);
}

(As an afterthought, I also tacked the entire program on the
end of this post, for anyone who's interested.)

I have two questions:

1. I had a "\" before the "$" to prevent "$_" from being
interpolated. But when I took the "\" out, the regexes
still worked fine! Seems to me they should break, because
$_ is now a variable rather than just "dollar sign followed
by underscore". But $_ seems not to be interpolated.
So, is variable interpolation always strictly "one pass"?

2. I've read that qr{} "compiles" the regex; I'm hoping that
means that the s/// operators in the while loop will not
recompile $Regex1 and $Regex2 each iteration, even though
I didn't use a /o flag? (No sense wasting CPU time
recompiling, because the patterns are fixed.)

Thanks in advance for your input!


===============================================================
IF YOU'RE PRESSED FOR TIME, FEEL FREE TO STOP READING HERE.
THE REMAINDER OF THIS POST IS THE WHOLE PROGRAM, FOR REFERENCE.
===============================================================


#!/usr/bin/perl

# linkify.perl

# Converts any text document into an HTML document with all of the contents of
# the original, but with any HTTP URLs converted to clickable hyperlinks.

# First print the standard opening lines of an HTML file.
# The title will be "Linkifyed HTML Document",
# the body text is in a "div" element,
# and the paragraphs will have 5-pixel margins on all 4 sides:

use strict;
use warnings;

# Print standard opening boilerplate crap for an HTML file:
print ("<html>\n");
print ("<head>\n");
print ("<title>Linkifyed HTML Document</title>\n");
print ("<style>p</style>\n");
print ("</head>\n");
print ("<body>\n");
print ("<div>\n");

# A valid URL must consist solely of the following 82 characters
#
# alphanumeric: [:alnum:] 62
# reserved: ;/?:@=& 7
# anchor-id: # 1
# encoding: % 1
# special: $_.+!*'(),- 11
# Total: 82
#

# Make a non-interpolated string version of a character class
# consisting of the above 82 URL-legal characters:
my $Legal = q;

# This regex says "find a string which is probably a URL minus the 'http://'
# part; save any such found string as a backreference":
my $Regex1 = qr

# This regex says "http or shttp or https or shttps, followed by '://',
# followed by a cluster of URL-legal characters; save any such found string
# as a backreference":
my $Regex2 = qr;

# Now loop through all lines of text in the original file, wrapping all URLS
# found in "a" and "p" elements, with the URL used as both the text and the
# "href" attribute of the "a" element:

while (<>)
{
# Linkify all http URLs, including the less-common "shttp" and "https" ones.

# This substitution says "tack 'http://' onto be beginning of any strins
# which are probably URLS sans 'http://':
sg;

# This substitution says "replace each found URL with an html anchor element
# with the found URL used both as the "href" atttribute and as the text,
# insert the anchor element into a paragraph element,
# and bracket the paragraph element with newlines":
s{\n<p><a href="$1">$1</a></p>\n}g;

# Print the edited line. If the line did not contain a URL, it will be
# printed unexpurgated. To redirect output to a file, use ">" on the
# command line.
print ($_);
}

# Print element-closure tags for div, body, html:
print ("</div>\n");
print ("</body>\n");
print ("</html>\n");

--
Cheers,
Robbie Hatley
lonewolf aatt well dott com
www dott well dott com slant user slant lonewolf slant



Posted by kens on April 13, 2008, 9:31 pm
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> Today I was editing a URL-likifying program I wrote several
> weeks ago, and I ran across some issues with q{} and qr{}
> which are puzzling me.
>
> Here's an edited-for-brevity version of the program:
>
> my $Legal = q;
> my $Regex1 = qr
> my $Regex2 = qr;
> while (<>)
> {
> sg;
> s{\n<p><a href="$1">$1</a></p>\n}g;
> print ($_);
>
> }
>
> (As an afterthought, I also tacked the entire program on the
> end of this post, for anyone who's interested.)
>
> I have two questions:
>
> 1. I had a "\" before the "$" to prevent "$_" from being
> interpolated. But when I took the "\" out, the regexes
> still worked fine! Seems to me they should break, because
> $_ is now a variable rather than just "dollar sign followed
> by underscore". But $_ seems not to be interpolated.
> So, is variable interpolation always strictly "one pass"?

q{} is equivalent to the single-quote operator. Strings inside single
quotes do not get interpolated (as opposed to double quotes - "" or
qq{}.

>
> 2. I've read that qr{} "compiles" the regex; I'm hoping that
> means that the s/// operators in the while loop will not
> recompile $Regex1 and $Regex2 each iteration, even though
> I didn't use a /o flag? (No sense wasting CPU time
> recompiling, because the patterns are fixed.)
>
Based on the documentation (perldoc perlop), qr may invoke a
precompilation of the pattern. To me that implies that it is
implementation specific, but there are others with more expertise in
this area than me.

HTH, Ken

> lines deleted

>
> --
> Cheers,
> Robbie Hatley
> lonewolf aatt well dott com
> www dott well dott com slant user slant lonewolf slant


Posted by John W. Krahn on April 13, 2008, 11:01 pm
Please log in for more thread options
Robbie Hatley wrote:
> Today I was editing a URL-likifying program I wrote several
> weeks ago, and I ran across some issues with q{} and qr{}
> which are puzzling me.
>
> Here's an edited-for-brevity version of the program:
>
> my $Legal = q;
> my $Regex1 = qr
> my $Regex2 = qr;
> while (<>)
> {
> sg;
> s{\n<p><a href="$1">$1</a></p>\n}g;
> print ($_);
> }
>
> (As an afterthought, I also tacked the entire program on the
> end of this post, for anyone who's interested.)
>
> I have two questions:
>
> 1. I had a "\" before the "$" to prevent "$_" from being
> interpolated.

That just adds a '\' character to your character class:

$ perl -le'$x = q; print qr'
(?-xism:[$_])
$ perl -le'$x = q; print qr'
(?-xism:[$_])

Which it doesn't look like you intended to include.

> But when I took the "\" out, the regexes
> still worked fine! Seems to me they should break, because
> $_ is now a variable rather than just "dollar sign followed
> by underscore". But $_ seems not to be interpolated.
> So, is variable interpolation always strictly "one pass"?

Read the "Gory details of parsing quoted constructs" section of:

perldoc perlop

> 2. I've read that qr{} "compiles" the regex; I'm hoping that
> means that the s/// operators in the while loop will not
> recompile $Regex1 and $Regex2 each iteration,

That is correct.

> even though
> I didn't use a /o flag? (No sense wasting CPU time
> recompiling, because the patterns are fixed.)

perldoc -q /o



John
--
Perl isn't a toolbox, but a small machine shop where you
can special-order certain sorts of tools at low cost and
in short order. -- Larry Wall

Posted by Robbie Hatley on April 14, 2008, 4:35 am
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"John W. Krahn" wrote:

> Robbie Hatley wrote:
>
> > is variable interpolation always strictly "one pass"?
>
> Read the "Gory details of parsing quoted constructs" section of:
> perldoc perlop

Thanks for the tip, but that section doesn't actually say
whether Perl variable interpolation is single-pass or
multi-pass (recursive).

However, when I scrolled up from that section, I noticed
that one of the sections above that:
http://perldoc.perl.org/perlop.html#Quote-and-Quote-like-Operators
*does* specify what I was looking for. It says:

"Perl does not expand multiple levels of interpolation."

Bingo. That's what I was wondering. That explains why "$_"
wasn't being interpolated in my program.

perl -le 'my $Cat=q/Fifi/; my $Dog=q/$Cat/; print qq/$Dog/;'

Prints "$Cat", not "Fifi" as I had expected. Now that I
understand why, I can avoid being surprised by that.

--
Cheers,
Robbie Hatley
perl -le 'print "4o6e7o4f0w5llc7m"'
perl -le 'print "0ttp//7ww.7ell.3om/~4onewolf/"'



Posted by Dr.Ruud on April 14, 2008, 6:33 pm
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John W. Krahn schreef:
> Robbie Hatley wrote:

>> 1. I had a "\" before the "$" to prevent "$_" from being
>> interpolated.
>
> That just adds a '\' character to your character class:
>
> $ perl -le'$x = q; print qr'
> (?-xism:[$_])
> $ perl -le'$x = q; print qr'
> (?-xism:[$_])

But it won't match a '\':

$ perl -wle'$x = q; print $x; print length($x); print q =~
/$x/ ? 1 : 0'
[$_]
5
0

$ perl -wle'$x = q; print $x; print length($x); print q =~
/$x/ ? 1 : 0'
[$_]
5
0

$ perl -wle'$x = q; print $x; print length($x); print q =~
/$x/ ? 1 : 0'
[$_]
6
1

$ perl -wle'$x = q; print $x; print length($x); print chr(92) =~
/$x/ ? 1 : 0'
[$_]
5
0

$ perl -wle'$x = q; print $x; print length($x); print chr(92) =~
/$x/ ? 1 : 0'
[$_]
5
0

$ perl -wle'$x = q; print $x; print length($x); print chr(92)
=~ /$x/ ? 1 : 0'
[$_]
6
1

(was run with a perl 5.8.5)

--
Affijn, Ruud

"Gewoon is een tijger."


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