I write a compiled regex as following, 
$pat = '(' . join('|', @patterns) . ')';
$pat = qr/$pat/i;

[download]
I then later add the 'g' modifier as this regex will be used in substituion that is expected to substitute all occurences, as in 
$pat = qr/$pat/gi;
s/$pat/something/;

[download]
But I got a syntax error near "$pat = qr/$pat/gi", so I move the 'g' on the substitution construct. 
$pat = qr/$pat/i;
s/$pat/something/g;

[download]
which is just fine. I wonder if this might be because 'g' is the property of a substitution, not a pattern match. I looked at the perlop and I did see the entry "qr/STRING/imosx", there's no 'g' there. However, this doesn't fit to my current undertanding about the 'g' usage, so I tried the ordinary matching construct, 
perl -e 'print if /perl/gi'

[download]
It went OK, and this is not supprising me since I use this kind of construct many times before: 
$str = 'nothing but perl can parse Perl';
@match = $str =~ /(perl)/gi;
print for @match;

[download]
I'm aware that qr// is only compilation and m// is compilation and execution at once. But, why 'g' doesn't work in qr//? Does it have anything to do with the way the regex is precompiled?

I thought that when perl precompiled a qr/regex/g, it noticed the g and planned (if this term is correct) to do the multimatching whenever the regex is later used, either in normal matching or substitution. I might overlook any explanation in the perlop and perlre and perlfaq6, or anywhere else about this, so where can I find it? I expected something in the perlop that says something like "you can't use g here because bla bla ....".

What I need is I compile a multimatch regex once and I use it in many substitution constructs without ever worrying about the 'g' modifier anymore.

In reply to The 'g' modifier in compiled regex by naikonta

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.