Python: String to Regex Pattern

Question

Suppose I have a string of characters.

charstr = "SZR"

Suppose that the character Z is a loaded character and can represent S, P, Q, W, or R

I want to write a function get_regex(charstr) which takes charstr as an input and returns a regular expression string. Which can then be used to find patterns in other strings.

The following code should not results in answer = 'None', since SZR matches SRR and SWR, which are in SQSRRSWR.

charstr = "SZR"
answer = re.search(get_regex(charstr), 'SQSRRSWR')

I have tried the following but it won't work. Any suggestions?

import re
def get_regex(charstr):
    charstr = re.sub("Z", "[SPQWR]{1}", charstr) # Z: can be S,P,Q,W, or R
    #The line directly below this was in my original post.  I have commmented it out and the function now works properly.
    #charstr = "\'\'\'^ " + charstr + "\'\'\'"    # Results in '''^ S[SPQWR]{1}R'''
    return charstr

charstr = "SZR"
answer = re.search(get_regex(charstr), 'SQSRRSWR')
print(answer)                                    # Results in None

commenting out the line that sets charstr to '''^ S[SPQWR]{1}R''' makes your regex work. Is there any significance to that part? — Wondercricket
– Wondercricket, Commented Nov 9, 2017 at 17:15
You have a space after the caret, plus you're including the single quotes in the string. Why? The regex string you're after is ^S[SPQWR]R, so why not just '^{}'.format(charstr)? — BallpointBen
– BallpointBen, Commented Nov 9, 2017 at 17:20
I thought you had to specify a regular expression in that way. Thanks. I have commented out that line in my code to reflect your suggestion. — DeeeeRoy
– DeeeeRoy, Commented Nov 9, 2017 at 17:22

sniperd · Accepted Answer · 2017-11-09 18:15:24Z

1

Your example seems to be pretty close to working. If I'm understanding what you are trying to do, this works:

import re

def get_regex(charstr):
    charstr = re.sub("Z", "[SPQWR]", charstr) # Z: can be S,P,Q,W, or R
    return charstr

charstr = "SZR"
if re.search(get_regex(charstr), 'SQSRRSWR'):
    print("yep it matched")
else:
    print("nope it does not match")

charstr = "SXR"
if re.search(get_regex(charstr), 'SQSRRSWR'):
    print("yep it matched")
else:
    print("nope it does not match")

results in:

yep it matched
nope it does not match

Which looks to be what you are trying to do. I took out the {1} since that is implied. Just toss a comment in if it doesn't seem right and I'll update this answer.

answered Nov 9, 2017 at 18:15

sniperd

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