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    Need help in regular expression: My first post in this forum


    need some help in the below regex. I am going through one site for learnign regular expressins. So far it has been going good
    but kind of stuck with below example. ( I want to know how regex works on token by token basis)

    the site says



    Let's take the regex <([A-Z][A-Z0-9]*)[^>]*>.*?</\1> without the word boundary and look inside the regex engine at the point where \1 fails the first time. First, .*? continues to expand until it has reached the end of the string, and </\1> has failed to match each time .*? matched one more character.

    Then the regex engine backtracks into the capturing group. [A-Z0-9]* has matched oo, but would just as happily match o or nothing at all. When backtracking, [A-Z0-9]* is forced to give up one character. The regex engine continues, exiting the capturing group a second time. Since [A-Z][A-Z0-9]* has now matched bo, that is what is stored into the capturing group, overwriting boo that was stored before. [^>]* matches the second o in the opening tag. >.*?</ matches >bold<. \1 fails again.

    The regex engine does all the same backtracking once more, until [A-Z0-9]* is forced to give up another character, causing it to match nothing, which the star allows. The capturing group now stores just b. [^>]* now matches oo. >.*?</ once again matches >bold<. \1 now succeeds, as does > and an overall match is found. But not the one we wanted.




    String is <boo>bold</b> and regex is <(A-Z][A-Z0-9]*)[^>.*?</\1>

    I know iam getting it wrong, however acc to my understanding , and the articles i have read on regex so far, i felt the regex should have worked like below......Please correct me

    Regex ---Token String
    <([A-Z][A-Z0-9]*)[^>].*?</\1> <boo>bold




    1) < consumes <

    2) [A-Z] in round bracket consumes b

    3) [A-Z0-9]* in round bracket consumes oo


    therefore , first backreference stores boo


    4) ^> doesnot match >


    Since the above token has star, so it is ok and we proceed to next token of regex. The position of string remains same


    5) > consumes > (which is first one in the string)

    6) .*? lazy Regex Engine will skip this token as . is lazy

    7) < doesnot match b

    so engine backtracks to pt 6, and . consumes b. similarly backtracking occurs over and over and . consumes "bold"

    8)< consumes < (which is second one)

    9)\1 which i think like mentioned in point 3, it must have value boo

    therefore
    boo doesnot match b

    10)So engine will backtrack to point 6 and now . will consume "bold<"

    11)< doesnot match \b

    so enigne backtracks and i guess . will now consume "bold<\b"





    but somehow its getting confusing from here ...Could anyone please help...The site mentioned below explains something else....iam unable to get it.....Thanks for your patience in advance
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  3. Sarcky
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    The RegExp Power Toy will show you precisely how the steps are laid out.

    Read the rest of the "resources" thread at the top of this forum for more.

    -Dan

    Comments on this post

    • drgroove agrees : Regex Power Toy... nice ref :D thx!
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