note
pileswasp
Woo. This one's got me interested.<BR><BR>
I've tested this on perl 5.004_04 for sun-solaris, perls 5.004_05 and 5.6 for i686-linux (redhat) and even ActiveState's 5.6.0 for Win32 and _all_ of them show the same behaviour.<BR>
What causes the difference between two variations on this bit of code is whether or not the pattern is plain text (as it says above <TT>/blah/ may be optimized to an analogue of index()</TT>). If there's no regex compilation then $& causes Segmentation faults.<BR><BR>
Using<BR><CODE>use re 'debug';</CODE><BR>shows that the regex isn't re-evaluated when the $& is entered on STDIN, but it does state explicitly <TT>Omitting $` $& $' support.</TT> Must say I'm at a bit of a loss as to where the value does come from.<BR><BR>
If I were to go out on a limb a bit I would say that I'm thinking that maybe the penalty from using $&, etc in your
code is because perl links it into plain text matches as well as compiled regexes. ie $&, etc are <EM>always</EM>
there for full compiled regex's, but <CODE>index()</CODE> doesn't normally return the pre-match, match and post-match strings, so the "analogue of index()" requires a bit more work to produce them.<BR><BR>
Where's japhy? I get the feeling he'll know :o)<BR><BR>
There's a bunch of tests and <TT>re 'debug'</TT> output below if you're interested:
<CODE><READMORE></CODE>
<CODE>
use re 'debug';
'foo' =~ m/.*/;
print eval <STDIN>;
</CODE>
This gives the following output:<BR>
<PRE>Compiling REx `.*'
size 3 first at 2
1: STAR(3)
2: REG_ANY(0)
3: END(0)
anchored(MBOL) implicit minlen 0
Omitting $` $& $' support.
EXECUTING...
Matching REx `.*' against `foo'
Setting an EVAL scope, savestack=3
0 <> <foo> | 1: STAR
REG_ANY can match 3 times out of 32767...
Setting an EVAL scope, savestack=3
3 <foo> <> | 3: END
Match successful!
</PRE>
Before waiting for the input. It actually specifies that it's omitting $&, etc support, yet when you do enter <STRONG>$&</STRONG> still gives the expected answer:
<PRE>Freeing REx: `.*'
foo
</PRE>
If you use a plain text match (like tilly suggested with /ri/ in 'string', you don't get this result at all, as perl
doesn't handle the match in the same way, it "guesses" the result, presumably using a more index() like way of making the match:<BR>
<CODE>use re 'debug';
'foo' =~ m/o/;
print eval <STDIN>;
</CODE>
gives the output:
<PRE>$ <STRONG>perl reg</STRONG>
Compiling REx `o'
size 3 first at 1
rarest char o at 0
1: EXACT <o>(3)
3: END(0)
anchored `o' at 0 (checking anchored isall) minlen 1
Omitting $` $& $' support.
EXECUTING...
Guessing start of match, REx `o' against `foo'...
Found anchored substr `o' at offset 1...
Guessed: match at offset 1
<STRONG>$&</STRONG>
Segmentation fault (core dumped)
</PRE>
<STRONG>$`</STRONG> and <STRONG>$'</STRONG> don't have quite such drastic efects, they simply print blank.<BR>
The extra level of compilation that look(ahead|behind)s give the regex also
allow $& to produce the required result:<BR>
<CODE>use re 'debug';
'foo' =~ m/(?<=f)o(?=o)/;
print eval <STDIN>;
</CODE>
Giving:
<PRE>$ <STRONG>perl reg</STRONG>
Compiling REx `(?<=f)o(?=o)'
size 15 first at 1
rarest char o at 0
1: IFMATCH[-1](7)
3: EXACT <f>(5)
5: SUCCEED(0)
6: TAIL(7)
7: EXACT <o>(9)
9: IFMATCH[-0](15)
11: EXACT <o>(13)
13: SUCCEED(0)
14: TAIL(15)
15: END(0)
anchored `o' at 0 (checking anchored) minlen 1
Omitting $` $& $' support.
EXECUTING...
Guessing start of match, REx `(?<=f)o(?=o)' against `foo'...
Found anchored substr `o' at offset 1...
Guessed: match at offset 1
Matching REx `(?<=f)o(?=o)' against `oo'
Setting an EVAL scope, savestack=3
1 <f> <oo> | 1: IFMATCH[-1]
0 <> <foo> | 3: EXACT <f>
1 <f> <oo> | 5: SUCCEED
could match...
1 <f> <oo> | 7: EXACT <o>
2 <fo> <o> | 9: IFMATCH[-0]
2 <fo> <o> | 11: EXACT <o>
3 <foo> <> | 13: SUCCEED
could match...
2 <fo> <o> | 15: END
Match successful!
<STRONG>$&</STRONG>
Freeing REx: `(?<=f)o(?=o)'
o
</PRE>
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