python get substring from regex

Question

I want to extract a substring from a string, which is conform to a certain regex. The regex is:

(\[\s*(\d)+ byte(s)?\s*\](\s*|\d|[A-F]|[a-f])+)

Which effectively means that all of these strings get accepted:

[4 bytes] 66 74 79 70 33 67 70 35
[ 4 bytes ] 66 74 79 70 33 67 70 35
[1 byte] 66 74 79 70 33 67 70 35

I want to extract only the amount of bytes (just the number) from this string. I thought of doing this with re.search, but I'm not sure if that will work. What would be the cleanest and most performant way of doing this?

score 6 · Accepted Answer · 2015-04-21 08:04:00Z

6

Use match.group to get the groups your regular expression defines:

import re

s = """[4 bytes] 66 74 79 70 33 67 70 35
[ 4 bytes ] 66 74 79 70 33 67 70 35
[1 byte] 66 74 79 70 33 67 70 35"""
r = re.compile(r"(\[\s*(\d)+ byte(s)?\s*\](\s*|\d|[A-F]|[a-f])+)")

for line in s.split("\n"):
    m = r.match(line)
    if m:
        print(m.group(2))

The first group matches [4 bytes], the second only 4.

Output:

4
4
1

edited Apr 21, 2015 at 8:04

answered Apr 21, 2015 at 8:00

user1907906

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4 Comments

Docteur Over a year ago

OP only wants the number of bytes!

nhahtdh Over a year ago

@pidgey (\s*|\d|[A-F]|[a-f])+ should be rewritten as [\s\dA-Fa-f]+ seeing as you don't care about what it captures.

user1907906 Over a year ago

@nhahtdh Please comment the question about the regex.

nhahtdh Over a year ago

@Tichodroma: I leave it here so that you can consider to include it in your answer. You can leave it alone if you don't want to. I have already pinged the OP in my comment.

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