Update: Nicely re-worked. Previously it didn't work but now it does. Nice job. I'm leaving my initial response here anyway, but it's no longer pertinent.

That doesn't work because it incorrectly matches invalid strings.

If you change 

my $str="abc";

[download]
to 
my $str="aaa";

[download]
You'll still get a match even though that's not supposed to be a valid string. You'll have to go a bit more complicated in the regex to exclude the false hits.

Update: I was missing my 2nd code snip.
--
I used to drive a Heisenbergmobile, but every time I looked at the speedometer, I got lost.

In reply to Re^2: REgular expression to check the string that allows "a","b" and "c" to occur only once in any order. by KurtSchwind
in thread REgular expression to check the string that allows "a","b" and "c" to occur only once in any order. by isha

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.