The RE engine is a recursive search for ways to try to match the pattern to the string.

'x??' means that it notes that it has a decision, and first tries to match no x's. If it fails then it will backtrack, reconsider that decision, and try to match one x then go forward. If that fails, then you fail.

'x*?' is similar. It notes that it has a decision, and first tries to match no x's. If it fails then it will backtrack, try to match one x, and then try to make the same decision over again. (That is the difference between the * and the ?. The ? will try to match at most once then proceed. The * returns to trying to match the preceeding group, and therefore can match there an unlimited number of times.)

If that didn't make sense then pick up Mastering Regular Expressions by Friedl. He explains it better.

In reply to Re: Re: Re: Do nothing? or Do something important in a very obscure way? by Anonymous Monk
in thread Do nothing? or Do something important in a very obscure way? by BrowserUk

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.