The Snom C520 does not use the regex dial string pattern. You can define your dial plan string using this as an example
Example: Dials when UK number has been entered, or when 156 has been entered
1. matching rule: 0[1-9]xxxxxxxxx number starting with 0, preceded by a 10-digit number
2. matching rule: 156
3. dialing rule x+P
4. You must press the "Talk" speaker key for the rule to work. Wait for dial tone and then enter the number.
5 Once the number entered matches either of the rules "0[1-9]xxxxxxxxx|156" the C520 will dial the number "x+P"