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Re^4: Is it safe to use external strings for regexes? (infinite loops)

by LanX (Sage)
on Oct 11, 2021 at 21:27 UTC ( #11137434=note: print w/replies, xml ) Need Help??


in reply to Re^3: Is it safe to use external strings for regexes? (infinite loops)
in thread Is it safe to use external strings for regexes?

> Thus Perl allows such constructs, by forcefully breaking the infinite loop.

Thanks, that wasn't clear to me.

BUT I should have taken more care about the

> WARNING: Difficult material (and prose) ahead. This section needs a rewrite.

FWIW, the forced break can be seen with re 'debug'

D:\tmp>perl -Mre=debug -e"'foo' =~ m{ ( o? )* }x;" Compiling REx " ( o? )* " Final program: 1: CURLYX[0]{0,INFTY} (12) 3: OPEN1 (5) 5: CURLY{0,1} (9) 7: EXACT <o> (0) 9: CLOSE1 (11) 11: WHILEM[1/1] (0) 12: NOTHING (13) 13: END (0) minlen 0 Matching REx " ( o? )* " against "foo" 0 <> <foo> | 0| 1:CURLYX[0]{0,INFTY}(12) 0 <> <foo> | 1| 11:WHILEM[1/1](0) | 1| WHILEM: matched 0 out of 0..65535 0 <> <foo> | 2| 3:OPEN1(5) 0 <> <foo> | 2| 5:CURLY{0,1}(9) | 2| EXACT <o> can match 0 times out +of 1... 0 <> <foo> | 3| 9:CLOSE1(11) 0 <> <foo> | 3| 11:WHILEM[1/1](0) | 3| WHILEM: matched 1 out of 0..655 +35 | 3| WHILEM: empty match detected, t +rying continuation... <---- HERE + 0 <> <foo> | 4| 12:NOTHING(13) 0 <> <foo> | 4| 13:END(0) Match successful! Freeing REx: " ( o? )* " D:\tmp>

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

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