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Pattern Matching of Regular Expressions

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I’m studing this topic and I met problem to understand this example…
 
rexCompile("\\([a-z]+\\)\\.\\1")        => t
rexExecute("abc.bc")                    => t
rexExecute("abc.ab")                    => nil
 
It is not clear why the second case is nil and if I type:
 
rexCompile("\\([a-z]+\\)\\.\\1")        => t
rexExecute("abc.bc")                    => t
 
rexSubstitute( “debug: \\0" ) => debug: bc.bc
 
Why the result is bc.bc?
 
 \1 register should not point to abc?
 
Please, could you clarify me how it works? Where could I find accurate information as the cdsdoc is not clear on how registers work.
 
Giuseppe

 


Originally posted in cdnusers.org by Giuseppe Greco
Parents

  • In the first case, you're expecting that the \([a-z]+\) matching happens in isolation. It doesn't. What happens is that it tries to match the entire pattern, and so it will be looking for a combination of 1 or more a-z characters. followed by ".", followed by exactly the same combination. Since it can successfully do that if the \1 part ends up as "bc", then it will do that.

    If you'd have anchored the pattern:

    rexCompile("^\\([a-z]+\\)\\.\\1")

    then this would have forced it to match the complete sequence of characters before the ".", in which case only "abc.abc" would have matched.

    So as was stated previously, you're matching:

    blablahblaXXXX.XXXX

    where XXXX is a variable length sequence of a-z - but is the [u]same[/u] either side of the dot.

    Regards,

    Andrew.


    Originally posted in cdnusers.org by adbeckett
Reply

  • In the first case, you're expecting that the \([a-z]+\) matching happens in isolation. It doesn't. What happens is that it tries to match the entire pattern, and so it will be looking for a combination of 1 or more a-z characters. followed by ".", followed by exactly the same combination. Since it can successfully do that if the \1 part ends up as "bc", then it will do that.

    If you'd have anchored the pattern:

    rexCompile("^\\([a-z]+\\)\\.\\1")

    then this would have forced it to match the complete sequence of characters before the ".", in which case only "abc.abc" would have matched.

    So as was stated previously, you're matching:

    blablahblaXXXX.XXXX

    where XXXX is a variable length sequence of a-z - but is the [u]same[/u] either side of the dot.

    Regards,

    Andrew.


    Originally posted in cdnusers.org by adbeckett
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