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in reply to Re: about word boundary in RE (use re 'debug')
in thread about word boundary in RE

Good but seems not easily readable to me. If (.+\b) already match, why I need the \s?
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Re^3: about word boundary in RE (use re 'debug')
by hdb (Monsignor) on Apr 02, 2013 at 08:00 UTC

    \b is a zero width match. It does need the space to recognize a word boundary, but it does not consume it. And therefore you need to add a space to your pattern.

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      Sorry my mistake. I didn't understand that the \1 repeat the pattern. I just neglcted it. So it's clear to me now. The case is from website. Thanks!

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Re^3: about word boundary in RE (use re 'debug')
by Anonymous Monk on Apr 02, 2013 at 08:21 UTC

    Good but seems not easily readable to me.

    In that case, use a shorter string, associate the numbers from "Final program" against those on the right side , like 1: OPEN1 (3) 

    $ perl -Mre=debug -le " q/a a/ =~ /(.\b)\1/ "
Compiling REx "(.\b)\1"
Final program:
   1: OPEN1 (3)
   3:   REG_ANY (4)
   4:   BOUND (5)
   5: CLOSE1 (7)
   7: REF1 (9)
   9: END (0)
minlen 1
Matching REx "(.\b)\1" against "a a"
   0 <> <a a>                |  1:OPEN1(3)
   0 <> <a a>                |  3:REG_ANY(4)
   1 <a> < a>                |  4:BOUND(5)
   1 <a> < a>                |  5:CLOSE1(7)
   1 <a> < a>                |  7:REF1(9)
                                  failed...
   1 <a> < a>                |  1:OPEN1(3)
   1 <a> < a>                |  3:REG_ANY(4)
   2 <a > <a>                |  4:BOUND(5)
   2 <a > <a>                |  5:CLOSE1(7)
   2 <a > <a>                |  7:REF1(9)
                                  failed...
   2 <a > <a>                |  1:OPEN1(3)
   2 <a > <a>                |  3:REG_ANY(4)
   3 <a a> <>                |  4:BOUND(5)
   3 <a a> <>                |  5:CLOSE1(7)
   3 <a a> <>                |  7:REF1(9)
                                  failed...
   3 <a a> <>                |  1:OPEN1(3)
   3 <a a> <>                |  3:REG_ANY(4)
                                  failed...
Match failed
Freeing REx: "(.\b)\1"

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    Compare against a simpler pattern like 

    $ perl -Mre=debug -le " q/aa/ =~ /a\b/ "
Compiling REx "a\b"
Final program:
   1: EXACT <a> (3)
   3: BOUND (4)
   4: END (0)
anchored "a" at 0 (checking anchored) minlen 1
Guessing start of match in sv for REx "a\b" against "aa"
Found anchored substr "a" at offset 0...
Guessed: match at offset 0
Matching REx "a\b" against "aa"
   0 <> <aa>                 |  1:EXACT <a>(3)
   1 <a> <a>                 |  3:BOUND(4)
                                  failed...
   1 <a> <a>                 |  1:EXACT <a>(3)
   2 <aa> <>                 |  3:BOUND(4)
   2 <aa> <>                 |  4:END(0)
Match successful!
Freeing REx: "a\b"

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    Then check the definition of \b in perlre#Assertions, perlrequick

    Perl defines the following zero-width assertions: The word anchor \b matches a boundary between a word character and a non-word character \w\W or \W\w 
         $x = "Housecat catenates house and cat";
    $x =~ /\bcat/; # matches cat in 'catenates'
    $x =~ /cat\b/; # matches cat in 'housecat'
    $x =~ /\bcat\b/; # matches 'cat' at end of string

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    Basically your pattern can never match, just like this perl -Mre=debug -le " q/aa/ =~ /a\ba/ "

    there can never be a word boundary within a word by definition

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[d/l]
[select]
      Thanks for your patience. I think I need a little more time on understand the lines lately.
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