I am not much of a regex person but am looking for some help on crafting a
simple regex filter that can do the following criterias:

1. The dialed number is always a 10-digits number, i.e. an area code +
7-digits number (without any leading digit(s) of +1 and/or 1).

2. If the 4-th digit of the dialed number (after the area code) is a
non-zero number, do nothing. Otherwise, the regex filter will strip the
leading 3 digits (area code) as well as the last 2 digits number and this
will leave only 1-st 5-digits number (no area code). For instance, if the
10-digits dialed number is 0120123456, then the regex filter will return
01234. More examples followed:

10-digits input number     output number
---------------------      -------------
   4120123456                01234
   0121234567                0121234567
   1310156434                01564
   8231234567                8231234567
   0126543210                0126543210
Anyone? Thanks.