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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