$x should match the pattern Y(digit)(number)
Surely you mean Y(letter)(number)?

Try this: 

$x =~ /Y[A-Z]\d+[^\d\\]*$/

[download]
as in 
for  my $x ("\\\\files\\builds\\data\\M9998SBQCACSYD30401S", "\\\\file
+s\\builds\\data\\M9998SBQCACSAD30401S") {
    if($x =~ /Y[A-Z]\d+[^\d\\]*$/) {
        print "$x matches\n";
    } else {
        print "$x doesn't match\n";
    }
}

[download]
which yields: 
\\files\builds\data\M9998SBQCACSYD30401S matches
\\files\builds\data\M9998SBQCACSAD30401S doesn't match

[download]

([^\d\\] makes sure you matched the last number, and no backslash follows.)

In reply to Re^3: Hlelp regarding regex by bart
in thread Hlelp regarding 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.