print $_, m[\s*(\d{0,3}\b)?] 
    ? " contained '$1'" 
    : ' contained nothing that matched' 
    for '','1','1 ',' 1','12',' 12 ','123',' 123',' 123 ','1234','1 2 3';

Use of uninitialized value in concatenation (.) or string at (eval 21) line 1, <STDIN> line 19.
  contained ''
1  contained '1'
1   contained '1'
 1  contained '1'
12  contained '12'
 12   contained '12'
123  contained '123'
 123  contained '123'
 123   contained '123'
1234  contained ''
1 2 3  contained '1'