^[13][a-km-zA-HJ-NP-Z1-9]{25,34}$
will match a string that starts with either 1 or 3 and, after that, 25 to 34 characters of either a-z, A-Z, or 0-9, excluding l, I, O and 0 (not valid characters in a Bitcoin address).
^[13][a-km-zA-HJ-NP-Z1-9]{25,34}$
will match a string that starts with either 1 or 3 and, after that, 25 to 34 characters of either a-z, A-Z, or 0-9, excluding l, I, O and 0 (not valid characters in a Bitcoin address).