I'm writing a quazi calculator in Parse::RecDescent. It will function as a real calculator for a larger project I'm working on eventually, but only if I can figure this problem out ;)

My grammar so far is below. The problem is that it loops for a _really_ long time if you use ()'s in the string. And when I say long, I'm talking generating 2.8 megs of tracing data. And even then, it ends up ignoring it (as far as I can tell). My thought process was it shouldn't loop unless there are nested ()'s because the first ( should be consumed upon a successful match right?

Anyway here's my grammar. Can someone find what's wrong? 
    startrule   : lv1expr

    lv1expr     : lv2expr ADD lv1expr  {$return = $item[1] + $item[3]}
                | lv2expr SUB lv1expr  {$return = $item[1] - $item[3]}
                | lv2expr

    lv2expr     : lv3expr MULT lv2expr {$return = $item[1] * $item[3]}
                | lv3expr DIV lv2expr  {$return = $item[1] / $item[3]}
                | lv3expr

    lv3expr     : lv4expr EXP lv3expr  {$return = $item[1] ** $item[3]
+}
                | lv4expr

    lv4expr     : '(' startrule ')'    {$return = $item[2]}
                | NUM

    EXP         : '^'
    MULT        : '*'
    DIV         : '/'
    ADD         : '+'
    SUB         : '-'
    NUM         : /\d+(?:\.\d+)?/

[download]


Once bread becomes toast, it can never be bread again.

In reply to Parse::RecDescent Puzzlement by Android 18

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.