RegEx to match Bitcoin addresses?

[^OIl] matches any character that's not O, I or l. The problems in your regex are:

• You don't have a $ at the end, so it'd match any string beginning with a BC address.
• You didn't count the first character in your {27,34} - that should be {26,33}

However, as mentioned in a comment, a regex is not a good way to validate 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).

^[13][a-km-zA-HJ-NP-Z1-9]{25,34}$

bitcoin address is

• an identifier of 26-35 alphanumeric characters
• beginning with the number 1 or 3
• random digits
• uppercase
• lowercase letters
• with the exception that the uppercase letter O, uppercase letter I, lowercase letter l, and the number 0 are never used to prevent visual ambiguity.

^(bc1|[13])[a-zA-HJ-NP-Z0-9]{25,39}$

Based on the new address type Bech32