In order to match the regexp, Perl construct a special kind of construct called an finite automata (I don't know enough about the innards to tell if it's a deterministic or a nondeterministic one, but it doesn't really matter).

In order to construct it, it needs to know some things. A zero-or-more repeats is simple to construct, as is one-or more (they take very few nodes). X-repeats (where X is a known number) involve creating more nodes (on the order of X). But you have to know how many, or you can't construct the automata. And that's why it doesn't work..

As for your second question, you would simply replace (\d) with (\d\d) - there is no reason why \1 needs to be just one character...

In reply to Re: RegExp madness by matija
in thread RegExp madness by palandir

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.