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cexp (elisp-library)

Poorman's implementation of combined expressions. Combined expressions are combinations of regular expressions and balanced expressions. You can use cexp-search-forward for searching combined expressions. Some clumsy way of storing the match-data and the balanced expressions is provided.

Example: You can search for definitions in TeX-files via the cexp

\\def\\[[:alpha:]]\(#[0-9]\)*\!(^{.*}$\!)

The special construct \!(...\!) captures a balanced expression.

If applied to the TeX file

\def\mdo#1{{\def\next{\relax}\def\tmp{#1}\ifx\next\tmp\else\def\next{#1\mdo}\expandafter}\next}

The search via cexp-search-forward with the above cexp returns the limits for the following groups:

  1. The beginning and the end of the full match
  2. The limits of the match for the regular expression before the balanced expression, i.e. \def\mdo#1
  3. The limits of the captured group in the first regular expression, i.e., #1
  4. The limits of the balanced expression, i.e., {{\def\next{\relax}\def\tmp{#1}\ifx\next\tmp\else\def\next{#1\mdo}\expandafter}\next}

An example from emacs.stackexchange.com matching the content of @media in css files

Assme you want to highlight the content in parenteses and curly brackets behind @media entries of css files. Therefore you need to find @media followed by two balanced expressions and to identify the two balanced expressions.

/*... css above*/
/*tablet*/
@media (max-width: 800px){
    ._desktop-only {display:none !important;}
    ._tablet-only {display:initial;}
    ._mobile-only {display:none;}
}
/*mobile*/
@media (max-width: 480px){
     ._desktop-only {display:none;}
     ._tablet-only {display:none !important;}
     ._mobile-only {display:initial;} /* comment */
}
/*more css below...*/

You can use the following elisp expression:

(cexp-search-forward "@media *\\!(.*\\!) *\\!(.*\\!)")

which works just like re-search-forward with additional sexps.

If you run that elisp expression match data is set as follows:

  • (match-string 0): the overall match

      "@media (max-width: 480px){
        ._desktop-only {display:none;}
        ._tablet-only {display:none !important;}
        ._mobile-only {display:initial;} /* comment */
      }"
    
  • (match-string 1): the stuff before the first sexp "@media "

  • (match-string 2): the first sexp "(max-width: 800px)"

  • (match-string 3): the regular expression match within the first balanced expression, i.e. the match for .* within the match for the first \\!(.*\\!): "(max-width: 800px)"

  • (match-string 4): the stuff between the first and the second sexp ""

  • (match-string 5): the second balanced expression

      "{
       ._desktop-only {display:none !important;}
       ._tablet-only {display:initial;}
       ._mobile-only {display:none;}
      }"
    
  • (match-string 6): the match for .* within the second balanced expression, i.e., "{"

Match string 6 is right and perhaps most interesting. It shows the difference between the sub-match that matches a balanced expression (i.e. match string 5) and the match within the balanced expression (i.e. match string 6). The dot . only matches characters that are not new-lines and on the first line of the second balanced expression there is only the opening curly brace.

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