Let's suppose we have, this in an XML
<cs:CUSTOMER_ID ><cs:CUSTOMER_I D/>

and a pattern to match : ":CUSTOMER_ ID>([A-Za-z][A-Za-z0-9]*|)"

Really, I understand that between the second [] looks for alpharethmetic.
But I don't understand the first [] and after the OR operator '|'.
Has anyone any experience to decipher it?

Thank you in advance