The general class of problem you're trying to solve is called 'parenthesis balancing', and you can't do it with regular expressions.. they don't have the computing power.

Every regular expression defines a set of strings that match the expression. That set is called a 'regular set', and the most convenient way to describe a regular set is to use the corresponding regular expression. Computationally, you can find every member of a regular set using a state machine, also known as a 'finite automaton'. Therefore, regular expressions, regular sets, and finite automata get lumped together as different ways of looking at the same problem.

Balanced parentheses are more one step more complicated than a regular set. They're called a 'context-free grammar', or CFG. Again, the grammar defines a set of strings, this time called a 'context-free language' (CFL), and there's a certain type of machine that can find every string in a CFL: a pushdown automaton (PA).

There are two general ways to program a PA: recursive functions, or a while() loop with a stack. As far as computing power is concerned, they're identical.

For the problem at hand, you'll want a two-layer program consisting of a lexer (a state machine) and a parser (a pushdown automaton). The lexer breaks the input stream down into chunks called 'tokens', then the parser shuffles those tokens around to build a data structure called a 'syntax tree'. Then you walk through the syntax tree to produce output.

That's all doable, but it's more complicated than just using a regular expression. Unfortunately, that's as simple as it gets.

In reply to Re: Re: parsing question by mstone
in thread Re: parsing question by sliles

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.