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few doubts on regular expressions

Aritra

 Hi,

     I was studing the manual of regular expressions. I have 3 questions regarding it:

1) How rexExecute is resulting 't' in first case & 'nil' in second ? 

rexCompile("\\([a-z]+\\)\\.\\1")        => t
rexExecute("abc.bc")                    => t
rexExecute("abc.ab")                    => nil

2)How the rexSubstitute is able to result only the numaric '123' ?

Is the register \0 stores the entire pattern "abc123" & \1 stores only "123" ?  How exactly does register storing happen ?

rexCompile( "[a-z]+\\([0-9]+\\)" ) => t
rexExecute( "abc123" )             => t
rexSubstitute( "*\\0*" )            => "*abc123*"
rexSubstitute( "The matched number is: \\1" )
                                    => "The matched number is: 123"
 

 3)Can anyone explain the following synopsis ?

[synopis] rs : [meaning] A composite regular expression rs matches the longest match of r followed by a match for s.

  • 1) The pattern that you've described says one or more a-z followed by a dot, followed by the match before the dot

    The first rexExecute meets this, because bc is before the dot and after it. Note it matches just bc before the dot rather than abc because it is trying to match the overall pattern. If you'd anchored the pattern (by putting "^" at the beginning say, then both would have returned nil. The second does not match because "ab" is not immediately before the dot.

    2) What you describe is correct; \0 is the entire match, whereas \1 is the first match in parentheses. If there was a second set of parentheses, that would end up in \2 and so on.

    3) In general matches with wildcards try to match the longest pattern it can (whilst trying to match the overall pattern). If there's an ambiguity though, the first pattern in a compound pattern will end up being as long as possible. An example of what I mean:

    rexCompile(".*a")
    rexExecute("helloaworldandaattheend")
    rexSubstitute("\\0") => "helloaworldandaa"

    The match has made the wildcarded part as long as it can, rather than just coming up with the result "helloa" or "helloaworlda" etc.

    Regards,

    Andrew.

  •  Hi Andrew,

                      Thanks for the explanation. I have 2 questions regarding it:

    1) In the example of wildcards [no 3 explanation], what exactly does the '*' signify ? I found that '+' also does the same job. Are they both same ?

    rexCompile(".+a")
    rexExecute("helloaworldandaattheend")
    rexSubstitute("\\0") => "helloaworldandaa"

    2) How to match the pattern that starts with an alphabet , then can have any thing & end with a perticular extension [say .il]. Like :

    test_2.il , A_b_v3.il [not _123.il ]

    I am unable to construct the middle part [which can have any thing ].

     

  • 1) * means any number (including 0) of the character that preceeds it. "." means any character. + means one or more of the character that preceeds it.

    2) "^[a-zA-Z].*\\.il$"

    Andrew.

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