Just use grouping, an alternation and a negative lookahead 
my $pat = qr/
  SW
  (?: # the grouping + alternation
      [:]
    |
      =? (?!ITCH) # ignore SWITCH
  )
  \s*
  ([^\s;,\n\r]+)
/x;

my %tests = (
  'SW: foo' => 1,
  'SW= bar' => 1,
  'SW  baz' => 1,
  'SWITCH'  => 0,
);

while(my($test,$res) = each %tests) {
  print "result: ", ($res ? $1 : 'PASS'), "\n"
    if $res == $test =~ $pat;
}

__output__

result: PASS
result: baz
result: foo
result: bar

[download]
See. perlre for more info.
HTH

_________
broquaint

In reply to Re: combining RegEx by broquaint
in thread combining RegEx by Anonymous Monk

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.