Regular expressions

[Denis McMahon]

On Mon, 02 Nov 2015 22:17:49 -0500, Seymore4Head wrote:

> On Mon, 2 Nov 2015 20:42:37 -0600, Tim Chase
> <python.list at tim.thechases.com> wrote:
> 
>>On 2015-11-02 20:09, Seymore4Head wrote:

>>> How do I make a regular expression that returns true if the end of the
>>> line is an asterisk

>>Why use a regular expression?

> Because that is the part of Python I am trying to learn at the moment.

The most important thing to learn about regular expressions is when to 
use them and when not to use them.

Returning true if the last character in a string is an asterisk is almost 
certainly a brilliant example of when not to use a regular expression. 
Here are some timings I tested:

#!/usr/bin/python

import re

import timeit

patt = re.compile("\*$")

start_time = timeit.default_timer()
for i in range(1000000):
    x = re.match("\*$", "test 1")
elapsed = timeit.default_timer() - start_time
print "re, false", elapsed

start_time = timeit.default_timer()
for i in range(1000000):
    x = re.match("\*$", "test *")
elapsed = timeit.default_timer() - start_time
print "re, true", elapsed

start_time = timeit.default_timer()
for i in range(1000000):
    x = patt.match("test 1")
elapsed = timeit.default_timer() - start_time
print "compiled re, false", elapsed

start_time = timeit.default_timer()
for i in range(1000000):
    x = patt.match("test *")
elapsed = timeit.default_timer() - start_time
print "compiled re, true", elapsed

start_time = timeit.default_timer()
for i in range(1000000):
    x = "test 1"[-1] == "*"
elapsed = timeit.default_timer() - start_time
print "char compare, false", elapsed

start_time = timeit.default_timer()
for i in range(1000000):
    x = "test *"[-1] == "*"
elapsed = timeit.default_timer() - start_time
print "char compare, true", elapsed

RESULTS:

re, false 2.4701731205
re, true 2.42048001289
compiled re, false 0.875837087631
compiled re, true 0.876382112503
char compare, false 0.26283121109
char compare, true 0.263465881348

The compiled re is about 3 times as fast as the uncompiled re. The 
character comparison is about 3 times as fast as the compiled re.

-- 
Denis McMahon, denismfmcmahon at gmail.com