Re^5: On the regex pattern variable to be inserted into another (which recompile???)

> But how did you prove that $foo is recompiling?

I can't find any proof that anything is recompiled, even with //g.


D:\>perl -E"my $x; BEGIN{$x=qr/a.*u/}; use re 'debug'; my $str = 'aou'
+; say $str
 =~ /$x/"
1

D:\>perl -E"my $x; BEGIN{$x=qr/a.*u/}; use re 'debug'; my $str = 'aou'
+; say $str
 =~ /$x/g"
aou
[download]

Please compare the verbose output if you replace qr// with q//

D:\>perl -E"my $x; BEGIN{$x=q/a.*u/}; use re 'debug'; my $str = 'aou';
+ say $str
=~ /$x/"
Compiling REx "a.*u"
Final program:
   1: EXACT <a> (3)
   3: STAR (5)
   4:   REG_ANY (0)
   5: EXACT <u> (7)
   7: END (0)
anchored "a" at 0..0 floating "u" at 1..9223372036854775807 (checking 
+floating)
minlen 2
Matching REx "a.*u" against "aou"
Intuit: trying to determine minimum start position...
  doing 'check' fbm scan, [1..3] gave 2
  Found floating substr "u" at offset 2 (rx_origin now 0)...
... YADDA YADDA ...
[download]

So how do you know???

Cheers Rolf
_{(addicted to the Perl Programming Language :)

Wikisyntax for the Monastery}

update

And that's whats happening if you have more than one precompiled regex inside the match, like expected.

D:\>perl -E"my $x; BEGIN{$x=qr/a.*u/}; use re 'debug'; my $str = 'aou'
+; say $str
 =~ /$x$x/"
Compiling REx "(?^u:a.*u)(?^u:a.*u)"
Final program:
   1: EXACT <a> (3)
   3: STAR (5)
   4:   REG_ANY (0)
   5: EXACT <ua> (9)
   9: STAR (11)
  10:   REG_ANY (0)
  11: EXACT <u> (13)
  13: END (0)
anchored "a" at 0..0 floating "ua" at 1..9223372036854775807 (checking
+ floating)
 minlen 4
String shorter than min possible regex match (3 < 4)

Freeing REx: "(?^u:a.*u)(?^u:a.*u)"
[download]

FWIW:

This is perl 5, version 32, subversion 1 (v5.32.1) built for MSWin32-x
+64-multi-t
hread
[download]

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Re^6: On the regex pattern variable to be inserted into another (which recompile???) by NERDVANA (Priest) on Feb 11, 2022 at 13:39 UTC
Well, I take it back, and have updated my answers. Perl does special-case `/$foo/g` and `s/$foo/.../g` to avoid re-compiling. It does not optimize changing the anchor like `/\G$foo/gc`, which is the actual code from my module. The testing I did at the time was based on performance, rather than specifically tracing whether the compilation got invoked. It could be that evaling a coderef with the expanded regex text still performs better than referencing a compiled regex, but but I don't really have time right now to go back and benchmark it.	[reply] [d/l] [select]
Re^7: On the regex pattern variable to be inserted into another (which recompile???) by hv (Prior) on Feb 11, 2022 at 18:59 UTC
AFAIR Perl caches the previous stringification of the regexp, and compares that against the stringification of the new pattern to determine whether it needs recompiling. The stringification of a compiled pattern always captures those outside flags that affect compile-time so as to make this (and interpolation) safe, that's why you see for example: `% perl -E 'say qr{foo}i' (?^ui:foo) %` [download] The `/g` flag is not listed in the docs because it is not supported by `qr{}`: `% perl -E 'say qr{foo}g' Unknown regexp modifier "/g" at -e line 1, near "say " Execution of -e aborted due to compilation errors. %` [download] (I think that error message could probably be improved. Patches welcome. :)	[reply] [d/l] [select]