Question

If you compile a regex inside a function, and that function gets called multiple times, does Python recompile the regex each time, or does Python cache the compiled regex (assuming the regex doesn’t change)?

For example:

def contains_text_of_interest(line):
    r = re.compile(r"foo\dbar\d")  
    return r.match(line)

def parse_file(fname):
    for line in open(fname):
        if contains_text_of_interest(line):
           # Do something interesting

Answer 1

Actually, if you look at the code in the re module, the re.compile function uses the cache just as all the other functions do, so compiling the same regex over and over again is very very cheap (a dictionary lookup). In other words, write the code to be the most understandable or maintainable or expressive, and don’t worry about the overhead of compiling regexes.