Finding a Pattern in string

Question

I have few different types of strings and want to find a particular pattern or say first number coming after the pattern in the string.Following are the example strings.

str1 = mprage_scan_0006__002.nii
str2 = fgre3d_scan_0005__001+orig.HEAD
str3 = f3dgr2_scan_7_afni_009.nii
str4 = 2dfgre_scan_09_08.nii

I want to extract numbers after 'scan_?' in every string. if it's '007' or '09' or any other way, I want to extract only the number i.e '7', '9' etc..

i did try this but looks like my solution is not that flexible as its find the first number in the string instead of first number after 'scan_' pattern.

import re
a = re.findall(r'\d+', str[i])
scan_id = re.sub("^0+","",a[0])

int(str1.split('_scan_')[1].rsplit('_')[0], 10)

Sede
– Sede

2016-04-05 16:08:16 +00:00
Commented Apr 5, 2016 at 16:08 — Sede
– Sede, Commented Apr 5, 2016 at 16:08

pkacprzak · Accepted Answer · 2016-04-05 16:10:29Z

2

Try something like that using positive lookbehind assertion:

>>> import re
>>> 
>>> str1 = 'mprage_scan_0006__002.nii'
>>> str2 = 'fgre3d_scan_0005__001+orig.HEAD'
>>> str3 = 'f3dgr2_scan_7_afni_009.nii'
>>> str4 = '2dfgre_scan_09_08.nii'
>>> 
>>> pattern = r'(?<=scan_)0*(:?\d+)'
>>> 
>>> for s in [str1, str2, str3, str4]:
...     m = re.search(pattern, s)
...     print m.group(1)
... 
6
5
7
9
>>>

answered Apr 5, 2016 at 16:10

pkacprzak

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3 Comments

Jared Goguen Over a year ago

Is there a reason to use lookbehind?

pkacprzak Over a year ago

In this case probably it doesn't matter, but in general, it's more efficient not to capture something we don't want it the match, and lookbehind doesn't capture the assertion

Jared Goguen Over a year ago

It seems like it would be even more efficient to use a non-matching group then (by intuition, I don't have a reference for that).

Francesco · Accepted Answer · 2016-04-05 16:05:40Z

2

Try with this one

a = re.findall(r'scan_0*(\d+)', str[i])

What it does:

look for scan_
followed by zero or more zeroes
followed by one ore more digits, and it selects this last group with the parentheses

answered Apr 5, 2016 at 16:05

Francesco

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3 Comments

Keatinge Over a year ago

the * symbol means 0 or more in regex. So it doesn't check if its followed by one or more zeros, it checks if its followed by 0 or more zeros

Francesco Over a year ago

@Racialz Exactly what I said

Francesco Over a year ago

@Racialz I'm sorry, I can't find the history of my answer. If you do you will see that I did not edit that part. Anyway thank you for pointing out the correct behaviour of symbol * in regex

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