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How do I apply switches like /i or /g to a qr regexp?

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Contributed by ambrus on Jun 20, 2004 at 21:53 UTC
Q&A  > regular expressions


Description:

How do I apply switches to a precompiled (qr) regexp.

This code does not work:

$regexp = qr/^(get|post)\b/; "GET / HTTP/1.1"=~m/$regexp/i and print " +match\n";

Answer: How do I apply switches like /i or /g to a qr regexp?
contributed by ambrus

There are two knid of regep switches in perl.

The first kind is the swithes /imsx, which change the meaning of the regexp itself. The second is /gce, which only change what perl does with a match; this kind can only be applied to the whole regexp, not some part of it.

For swiches /imsx, you have to write these directly after the closing delimiter of the qr, because perl has to know about the switches when it compiles the regexp. Thus, to match case-insensitively, you'll need something like this:

$regexp = qr/^(get|post)\b/i; "GET / HTTP/1.1"=~m/$regexp/ and print " +match\n";
As you already know, if you use a single qr scalar in a m// expression, the regexp won't get recompiled. The m// or s/// is neccesarry only if you want to add one of the switches /gce. Even if you have other characters in the m//, the /imsx flags after m// does not apply to the regexp part inside the qr, so the following code won't match either as (get|post) is matched case-sensitively.
$regexp = qr@get|post@; "GET / HTTP/1.1"=~m@^$regexp\s+(\S+)@i and print "match\n";

If you stringify a qr regexp, like print qr/hallo/;, you'll see that it prints like (?-xism:hallo) showing that the flags are fixed in it, even if you embed it in a larger regex.

On the other hand, you cannot use any of the /gce switches in a qr like qr/^count:\s*(\d+)/g, as these make sense only with a m// or s///. Thus, to match a regexp from pos, you need to write

$regexp = qr/(\d+)/; while ($line=~/$regexp/g) { print "number $1 found\n"; }

This question is also answered in Friedl, Mastering Regualr Expressions, chapter 7 (p. 299 and p. 346 in the translation).

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