(edit) I have to now agree with the other posters, that my interpretation of the question is just one of many. The below is a simple answer to one of those interpretations.

I can't believe everyone's missed the obvious! 

$myvar =~ /X/i && $myvar !~ /[A-WYZ]/i

[download]

If you want to try putting it all in one regexp, see this meditation- Matching and nonmatching multiple regexps at once

And if you want it to be Unicode-safe, not just matching ASCII- I'm sure it can be and should be done, but I'm not the one to ask...

(edit #2) And if you really want to know if a string is a single letter X then don't use regexp. $myvar eq 'X'

In reply to Re: Check if a variable contains only one letter? by Yary
in thread Check if a variable contains only one letter? 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.