If you don't want the space in the substitution, don't include it in the capture group! 

$ perl -E'
my $re = qr/ \s ( \d{3,4} ) /x;
say ">$1<" if " 5678" =~ /$re/; 
'
>5678<

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The way forward always starts with a minimal test.

In reply to Re^3: Recognizing 3 and 4 digit number by 1nickt
in thread Recognizing 3 and 4 digit number by htmanning

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