Basically, the regex says 'May or may not start with 'a', but otherwise I don't care'. In other words, this regex will evaluate to TRUE with any value you pass it, since '*' signifies 'zero or many'.

If you are looking for a regex that says 'Must start with the character class I specify, otherwise fail', you will need to substitute the '*' for a '+', where the '+' implies 'one or many' rather than 'zero or many'.

Therefore: 
$foo = "1";
if ( $foo =~ /^[a]+/ ) {
  print "Match\n";
}

[download]
You will see the difference.

Also, a good resource for these types of questions are in the Perl documentation. For Perl Regular Expressions, try 'perldoc -f perlre' from the command-line, or Perlmonks has the page as well.

---------
perl -le '$.=[qw(104 97 124 124 116 97)];*p=sub{[@{$_[0]},(45)x 2]};*d=sub{[(45)x 2,@{$_[0]}]};print map{chr}@{p(d($.))}'

In reply to Re: Regex Semantics by wazzuteke
in thread Regex Semantics by cbro

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.