[0-9](?=[a-z]) will match any one digit followed by a lowercase letter.
[0-9](?![a-z]) will match any one digit NOT followed by a lowercase letter. (this is called a negative lookahead)
(?<=[a-z])[0-9] will match any one digit preceeded by a lowercase letter.
(?<![a-z])[0-9] will match any one digit NOT preceeded by a lowercase letter.(this is called a negative lookbehind)