Did you try changing \d{4} to \d{4,5}?

m[(\d{4,5})\.htm] and print $1 for qw[ xxx1234.htm xxxx12345.htm ];;
1234
12345
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If you need to exclude urls that might contain more than 4 or 5 digits:

m[(?<=\D)(\d{4,5})\.htm] and print $1 for qw[ xxx123.htm xxx1234.htm x
+xxx12345.htm xxx123456789.htm ];;
1234
12345
[download]