Here's my attempt which seems a little simpler than some of the others. 

m[^ \d (?: (?<! \x20 ) \d | \x20 ){19} $]x

[download]

Which says that the entire string must constist of a digit followed by 19 ((digits not preceeded by spaces) or spaces). 

print m[^ \d (?: (?<!\x20) \d | \x20 ){19} $]x 
    ? 'Yes:' . $_ 
    : ' No:' . $_ 
    for @t;

 No:     123451234512345
 No:  123451234512345
Yes:123451234512345
Yes:1234512345
 No:123 451 2345
 No:

[download]
Examine what is said, not who speaks.
"Efficiency is intelligent laziness." -David Dunham
"Think for yourself!" - Abigail
Hooray!

In reply to Re: Regex to match 20 chars of some digits followed by some spaces by BrowserUk
in thread Regex to match 20 chars of some digits followed by some spaces by leriksen

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.