$tags=~/(#\S+).+$1/;

Why is it a bad idea to compile a capture variable, e.g., $1, into a regex as haukex and LanX have noted?

Capture variables have the values they were assigned on execution of the most recent | most recent successful m// or s/// match, or undef if no match | no successful match has ever been done. 

Win8 Strawberry 5.8.9.5 (32)  Sat 07/23/2022 16:42:27
C:\@Work\Perl\monks
>perl
use strict;
use warnings;

use Data::Dump qw(dd);

dd 'A: $1', $1;

my $rx = qr/(#\S+).+$1/;
print "B: $rx \n";

'OOPS' =~ /(OOPS)/;
$rx = qr/(#\S+).+$1/;
print "C: $rx \n";
^Z
("A: \$1", undef)
Use of uninitialized value in concatenation (.) or string at - line 8.
B: (?-xism:(#\S+).+)
C: (?-xism:(#\S+).+OOPS)

[download]

In this example, $1 is undefined at A because no match that assigned a defined value to it has ever been done.

At B, a regex is compiled using the undefined $1. Compilation produces an "uninitialized value" warning, and the compiled regex has nothing where $1 was. This is because an undefined value is stringized as the empty string. This is a good example of why warnings (and strict!) should always be enabled. You make no mention of any warning message, so I assume you did not do this. I turn the arched eyebrow of scorn upon you.

At C, the same regex is compiled again after $1 has been given a defined value by a match. The result in this case is a potentially very problematic bug. Note that no warning message is generated! Good luck with this one.

The take-away: always use warnings and strict.

Give a man a fish:  <%-{-{-{-<

In reply to Re: Regex: remove non-adjacent duplicate hashtags (updated) by AnomalousMonk
in thread Regex: remove non-adjacent duplicate hashtags by element22

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.