I am using the module re in Python 3.3 to work with regular expressions. Users give my program a string (supposed to be regex) and it has to work with that regex.
I think it is pretty impossible, but, is there any way to know if a string is a valid regex?
Thank you!
You can simply try to compile the regex using re.compile(), and an re.error will be thrown if the regex is invalid.
try:
re.compile(regex)
except re.error:
# not valid
I think that trying to re.compile() and catch errors is the most useful way, but depending on how complicated the regex can be, it could also be possible to use a regex to test if a given string is a regex...
(But I think to describe all valid Python regexes, you need at least a context free grammar)
You can try compiling it within a try finally. This won't catch use case when a user accidentally makes a valid regex, but it will catch any instance when he supplies an invalid one. You can try replacing the invalid '++' string with something valid like '[1-9]' to see it work.
import re
try:
r = re.compile('++')
except:
print 'Invalid Regex'
else:
print 'Regex valid'
