If the alpha characters are always at the front, then a combination of the Extract and Strip function so do what you need.
Strip("ABCDEFGHIJKLMNOPQRSTUVWXYZ",[Client Account Number]) will result in the following values:
Embedding the strip within the following extract function will extract from the start of the string to whatever the strip function returns.
Extract([Client Account Number],””, Strip("ABCDEFGHIJKLMNOPQRSTUVWXYZ",[Client Account Number])).
There will no doubt be a more concise RegEx expression that will offer greater flexibility if this doesn’t do what you need. Please let me know and we can look into that if required.