Can a regular expression be used as a key in a dictionary?

Question

I want to create a dictionary where the keys are regular expressions:

d = {'a.*': some_value1, 'b.*': some_value2}

Then, when I look into the dictionary:

d['apple']

I want apple 'apple' to be matched against the keys which are regular expressions. If there is a complete match with a key/regular-expression then the corresponding value should be returned.

For example 'apple' matches with the regular expression 'a.*' completely, and so, some_value1 should be returned.

Of course, all of this assumes that the regular expression keys do not conflict (i.e. two keys should not both match the same string exactly). Let's say I can manually take care of this requirement when building my keys.

Is this possible in Python? If so, it would be quite an elegant and powerful construct!

Answer 1

Python dictionaries are implemented as hash tables - which means any mydict[myvalue] lookup is very fast by internally hashing myvalue. Using regular expressions as keys will cancel this functionality. Instead of using a dictionary, you should use a simple list or tuple where each item is a tuple in the format: (pattern/compiled regular expression, value) and scan them until a regular expression passes. This will also give you the ability to play with the order of regular expressions (from specific to general, for example):

import re

LOOKUPS  = [
    ('a.*', 'a'),
    ('b.*', 'b'),
]

def lookup(s, lookups):
    for pattern, value in lookups:
        if re.search(pattern, s):
            return value
    return None

print(lookup("apple", LOOKUPS))

See also Django's url resolver for a (very) advanced implementation of your idea.