hum, it does matches all of those

print(   "1  " =~ /\d{0,3}/   ?"match":"no match", $/);
print(   "12 " =~ /\d{0,3}/   ?"match":"no match", $/);
print(   "  1" =~ /\d{0,3}/   ?"match":"no match", $/);
print(   "   " =~ /\d{0,3}/   ?"match":"no match", $/);
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Now, if you want the regexp to completely match exactly three characters, consiting of 0+ digits padded with spaces on either side, that's much more complicated.

We can write it out the long way:
/^(?:   |  \d| \d\d|\d\d\d|\d  |\d\d |\d  )$/
This doesn't compress well.

/^[ \d]{3}$/
will match them all, but it will also match "1 2".

/^(?: [ \d]{2}|[ \d]{2} |\d{3})$/ will work.

Anything else I can think of invovles potentially matching more than three characters (/^\s*\d{0,3}\s*$/) or involves more than a regexp.

If the spaces can only be leading spaces, then the long form would be:
/^(?:   |  \d| \d\d|\d\d\d)$/
which doesn't really simplify.

btw, you should be using \D if you mean non-digit, or \S if you mean non-whitespace. \W doesn't appear to be appropriate here.