How to check if a string is a valid regex in Python?

Similar to Java. Use re.error exception:

import re

try:
    re.compile('[')
    is_valid = True
except re.error:
    is_valid = False

exception re.error

Exception raised when a string passed to one of the functions here is
not a valid regular expression (for example, it might contain
unmatched parentheses) or when some other error occurs during
compilation or matching. It is never an error if a string contains no
match for a pattern.