Thread: Regular expressions

  1. #1
    Registered User
    Join Date
    May 2006

    Regular expressions

    Hi there;

    I know what regular expressions are, and have used very simple ones before - but I don't know if they can be used in C, or how to use them. I do know, though, that regex would save me a heck of a lot of trouble in parsing a slightly regular string (tokenising).

    What I am doing, is checking for the following regular expression:


    Where there is a *, there can be any number of whitespace.

    I suppose I am going against cboard ettiquette here and asking for a bit of a write out, but I don't know where to start looking (as I asked before, I don't even know if regex can be used in C).

    Googling attempts seem to throw 1000s of C++ string regex functions at me but no C..


  2. #2
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    Just use a library - example
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
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    May 2006
    Regular expressions are nice, but I don't think I'd use a library to deal with whitespace. You could easily just use a C function like sscanf() that will ignore whitespace. Something like:

    const char line[] = "blah  123,     456,   789";
    char str[50];
    int i1, i2, i3;
    sscanf(line, "%s %d, %d, %d", str, &i1, &i2, &i3);
    printf("anystring: %s\n"
           "anyint1: %d\n"
           "anyint2: %d\n"
           "anyint3: %d\n", str, i1, i2, i3);
    or if you were reading from a file, you could use fscanf() which is the same thing, but it uses a file for its first parameter. In a way, C has functions that use a regular expression, like sscanf, but it's just not as complex as something like Perl style regex's. So I'd suggest looking more into the different string functions C. Anyway, good luck.

  4. #4
    Mad OnionKnight's Avatar
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    Jan 2005
    UmeŚ, Sweden
    $ in a regex is the end of line anchor. You sure that's what you want?

  5. #5
    Registered User
    Join Date
    May 2006
    hmm, I just figured he didn't know much about regular expressions and meant that those were variables. I think those *'s would more likely become \s+ if it was a real regular expression also.

  6. #6
    Registered User ssharish2005's Avatar
    Join Date
    Sep 2005
    Cambridge, UK
    * represents -> 0 or more elements Ex: set{E, a ,aa,aaa,aaaa,.....} where Ereprents empty string
    + represents -> 1 or more elements Ex: set{a,aa,aaa,aaaa,....}

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