in reply to Re^3: Composing regex's dynamically
in thread Composing regex's dynamically

Why are the dollar signs doubled?

I would not expect the -xism part to cost anything over the (?: ) without that. Again, that makes me wonder if re-compiling the string is worse than incorporating a compiled regex in another qr. Naturally, being able to leave off the parens completely simplifies things.

The regex engine has improved its optimizations in successive versions of Perl too; I think I read something about that in either the 5.10 or 5.12 notes.

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Re^5: Composing regex's dynamically
by JavaFan (Canon) on Apr 29, 2011 at 09:51 UTC
    Why are the dollar signs doubled?
    Typos. Fixed.
    Again, that makes me wonder if re-compiling the string is worse than incorporating a compiled regex in another qr.
    It's certainly not *worse*. Note that if you do: 
    my $re1 = qr/foo/;
my $re2 = qr/bar/;
my $re3 = qr/$re1|$re2/;

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    you compile three patterns, and stringify two. If you write: 
    my $re4 = qq/foo/;
my $re5 = qq/bar/;
my $re6 = qr/$re4|$re5/;

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    you compile only one pattern, and stringify none.
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      You are asserting that any time you interpolate a compiled re into another, it just stringifies the first anyway, and re-compiles the whole thing?

      I had always assumed that it copies the compiled bits and does not need to re-process it. I formed that opinion in the early days of the existence of qr, so that was a long time ago now.

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        You are asserting that any time you interpolate a compiled re into another, it just stringifies the first anyway, and re-compiles the whole thing?
        That's no assertion. That's how it works.
        I had always assumed that it copies the compiled bits and does not need to re-process it.
        No, that's not really possible to do (think for instance paren counts - or consider this: $q = qr/x/; $r = qr/[$q]/). It only works in this case: 
        my $qr1 = /pattern/;
my $qr2 = /$qr1/;

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        Use use re 'debug' and you will see what strings are being compiled.
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        What JavaFan said 
        $ perl -Mre=debug -e " $f = qr/./; warn if m/$f/"
Compiling REx "."
Final program:
   1: REG_ANY (2)
   2: END (0)
minlen 1
Freeing REx: "."

$ perl -Mre=debug -e " $f = qr/./; $g = qr/$f/; warn if m/$g/"
Compiling REx "."
Final program:
   1: REG_ANY (2)
   2: END (0)
minlen 1
Freeing REx: "."

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        $ perl -Mre=debug -e " $f = qr/./; $g = qr/./; $c=qr/$f$g/; "
Compiling REx "."
Final program:
   1: REG_ANY (2)
   2: END (0)
minlen 1
Compiling REx "."
Final program:
   1: REG_ANY (2)
   2: END (0)
minlen 1
Compiling REx "(?-xism:.)(?-xism:.)"
Final program:
   1: REG_ANY (2)
   2: REG_ANY (3)
   3: END (0)
minlen 2
Freeing REx: "."
Freeing REx: "."
Freeing REx: "(?-xism:.)(?-xism:.)"

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