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$ perl -E '
    say "$_: ", /1?\d{1,4}/ ? "Y" : "N"
...
0000: Y
19999: Y
999999999: Y

$ perl -E '
    say "$_: ", /(?<![0-9])(?:10000|[1-9][0-9]{0,3})(?![0-9])/ ? "Y" :
+ "N"
...
0000: N
19999: N
999999999: N