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Re: Why would one want in a regex a class with only a single entry?

by hipowls (Curate)
on Mar 25, 2008 at 07:53 UTC ( #676069=note: print w/ replies, xml ) Need Help??


in reply to Why would one want in a regex a class with only a single entry?

Character classes don't involve backtracking as this example shows.

my $string = 'bet'; use re qw(debug); my $rx1 = qr/b(?:a|e)t/; $string =~ /$rx1/; my $rx2 = qr/b[ae]t/; $string =~ /$rx2/;
output of $rx1 which shows the branch
Compiling REx `b(?:a|e)t' size 12 Got 100 bytes for offset annotations. first at 1 1: EXACT <b>(3) 3: BRANCH(6) 4: EXACT <a>(10) 6: BRANCH(9) 7: EXACT <e>(10) 9: TAIL(10) 10: EXACT <t>(12) 12: END(0) anchored "b" at 0 (checking anchored) minlen 3 Offsets: [12] 1[1] 0[0] 4[1] 5[1] 0[0] 6[1] 7[1] 0[0] 7[0] 9[1] 0[0] 10[0] Guessing start of match, REx "b(?:a|e)t" against "bet"... Found anchored substr "b" at offset 0... Guessed: match at offset 0 Matching REx "b(?:a|e)t" against "bet" Setting an EVAL scope, savestack=7 0 <> <bet> | 1: EXACT <b> 1 <b> <et> | 3: BRANCH Setting an EVAL scope, savestack=13 1 <b> <et> | 4: EXACT <a> failed... 1 <b> <et> | 7: EXACT <e> 2 <be> <t> | 10: EXACT <t> 3 <bet> <> | 12: END Match successful!
output of $rx2 where there is no branching.
Compiling REx `b[ae]t' size 16 Got 132 bytes for offset annotations. first at 1 1: EXACT <b>(3) 3: ANYOF[ae](14) 14: EXACT <t>(16) 16: END(0) anchored "b" at 0 (checking anchored) minlen 3 Offsets: [16] 1[1] 0[0] 2[4] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[0] 0[ +0] 6[1] 0[0] 7[0] Guessing start of match, REx "b[ae]t" against "bet"... Found anchored substr "b" at offset 0... Guessed: match at offset 0 Matching REx "b[ae]t" against "bet" Setting an EVAL scope, savestack=9 0 <> <bet> | 1: EXACT <b> 1 <b> <et> | 3: ANYOF[ae] 2 <be> <t> | 14: EXACT <t> 3 <bet> <> | 16: END Match successful! Freeing REx: `"b(?:a|e)t"' Freeing REx: `"b[ae]t"'
These examples were run under perl 5.8.8. 5.9.5 introduced trie optimization which gets rid of the branch. Output of $rx1 under 5.10.0
Compiling REx "b(?:a|e)t" Final program: 1: EXACT <b> (3) 3: TRIE-EXACT[ae] (10) <a> <e> 10: EXACT <t> (12) 12: END (0) anchored "b" at 0 (checking anchored) minlen 3 Guessing start of match in sv for REx "b(?:a|e)t" against "bet" Found anchored substr "b" at offset 0... Guessed: match at offset 0 Matching REx "b(?:a|e)t" against "bet" 0 <> <bet> | 1:EXACT <b>(3) 1 <b> <et> | 3:TRIE-EXACT[ae](10) 1 <b> <et> | State: 1 Accepted: 0 Charid: +2 CP: 65 After State: 3 2 <be> <t> | State: 3 Accepted: 1 Charid: +2 CP: 0 After State: 0 got 1 possible matches only one match left: #2 <e> 2 <be> <t> | 10:EXACT <t>(12) 3 <bet> <> | 12:END(0) Match successful! Freeing REx: "b(?:a|e)t"


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Re^2: Why would one want in a regex a class with only a single entry?
by ack (Deacon) on Mar 26, 2008 at 03:57 UTC

    Ah, of course! I mistakenly was thinking of the alternating classes rather than classes themselves. My mistake...and a novice one at that.

    Thanks, hipowls.

    ack Albuquerque, NM

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