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

Question

In Java, I could use the following function to check if a string is a valid regex (source):

boolean isRegex;
try {
  Pattern.compile(input);
  isRegex = true;
} catch (PatternSyntaxException e) {
  isRegex = false;
}

Is there a Python equivalent of the Pattern.compile() and PatternSyntaxException? If so, what is it?

Wouldn't any string be a valid regular expression? I could be wrong — TerryA
– TerryA, Commented Oct 28, 2013 at 9:22
@Haidro no, think of missing brackets and illegal use of special characters — Robin Krahl
– Robin Krahl, Commented Oct 28, 2013 at 9:23
is there a way i can use try...except and re.compile in python? will the re.compile validate regex? — alvas
– alvas, Commented Oct 28, 2013 at 9:24
@alvas Something like re.compile("(" * 1000 + "a" + ")"*1000) will fail with RuntimeError: maximum recursion depth exceeded instead of re.error — dbr
– dbr, Commented Oct 28, 2013 at 9:41

falsetru · Accepted Answer · 2013-10-28 09:24:55Z

108

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.

answered Oct 28, 2013 at 9:24

falsetru

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6 Comments

falsetru Over a year ago

@alvas, As long as you're using Regular expression supported by Python.

Tomasz Gandor Over a year ago

This is OK for regex syntax errors, but mind also TypeError if the pattern is not a str.

user202729 Over a year ago

And also RecursionError for regexes similar to this comment. It can also be raised for invalid regex, for example '('*1000+'a'+')'*1000+'('.

falsetru Over a year ago

@TomaszGandor, user202729, Thank you for the comment.

Belegnar Over a year ago

nevertheless, string{data} is valid regexp

|

Amit Gupta · Accepted Answer · 2020-09-17 14:46:47Z

6

Another fancy way to write the same answer:

import re
try:
    print(bool(re.compile(input())))
except re.error:
    print('False')

answered Sep 17, 2020 at 14:46

Amit Gupta

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