comment on

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/;
[download]

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!
[download]

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"'
[download]

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"
[download]

In reply to Re: Why would one want in a regex a class with only a single entry? by hipowls
in thread Why would one want in a regex a class with only a single entry? by ack

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