Python re match last underscore in a string

Question

I have some strings that look like this

S25m\S25m_16Q_-2dB.png
S25m\S25m_1_16Q_0dB.png
S25m\S25m_2_16Q_2dB.png

I want to get the string between slash and the last underscore, and also the string between last underscore and extension, so

Desired:

[S25m_16Q, S25m_1_16Q, S25m_2_16Q]
[-2dB, 0dB, 2dB]

I was able to get the whole thing between slash and extension by doing

foo = "S25m\S25m_16Q_-2dB.png"
match = re.search(r'([a-zA-Z0-9_-]*)\.(\w+)', foo)
match.group(1)

But I don't know how to make a pattern so I could split it by the last underscore.

falsetru · Accepted Answer · 2013-08-21 07:50:05Z

5

Capture the groups you want to get.

>>> re.search(r'([-\w]*)_([-\w]+)\.\w+', "S25m\S25m_16Q_-2dB.png").groups()
('S25m_16Q', '-2dB')
>>> re.search(r'([-\w]*)_([-\w]+)\.\w+', "S25m\S25m_1_16Q_0dB.png").groups()
('S25m_1_16Q', '0dB')
>>> re.search(r'([-\w]*)_([-\w]+)\.\w+', "S25m\S25m_2_16Q_2dB.png").groups()
('S25m_2_16Q', '2dB')

* matches the previous character set greedily (consumes as many as possible); it continues to the last _ since \w includes letters, numbers, and underscore.

>>> zip(*[m.groups() for m in re.finditer(r'([-\w]*)_([-\w]+)\.\w+', r'''
... S25m\S25m_16Q_-2dB.png 
... S25m\S25m_1_16Q_0dB.png
... S25m\S25m_2_16Q_2dB.png
... ''')])
[('S25m_16Q', 'S25m_1_16Q', 'S25m_2_16Q'), ('-2dB', '0dB', '2dB')]

edited Aug 21, 2013 at 7:50

answered Aug 21, 2013 at 7:15

falsetru

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3 Comments

LWZ Over a year ago

Thanks! How did the pattern differentiate different underscores? Where is the "last underscore" part?

LWZ Over a year ago

I see. Now I can understand this one. I guess I don't know re very well in general.

beroe Over a year ago

Also helps to know that the underscore is included by the \w wildcard.

TerryA · Accepted Answer · 2013-08-21 07:17:12Z

0

A non-regex solution (albeit rather messy):

>>> import os
>>> s = "S25m\S25m_16Q_-2dB.png"
>>> first, _, last = s.partition("\\")[2].rpartition('_')
>>> print (first, os.path.splitext(last)[0])
('S25m_16Q', '-2dB')

answered Aug 21, 2013 at 7:17

TerryA

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Comments

will · Accepted Answer · 2013-08-21 07:34:13Z

-3

I know it says using re, but why not just use split?

strings = """S25m\S25m_16Q_-2dB.png
S25m\S25m_1_16Q_0dB.png
S25m\S25m_2_16Q_2dB.png"""

strings = strings.split("\n")

parts = []
for string in strings:
    string = string.split(".png")[0] #Get rid of file extension
    string = string.split("\\")
    splitString = string[1].split("_")
    firstPart = "_".join(splitString[:-1]) # string between slash and last underscore
    parts.append([firstPart, splitString[-1]])


for line in parts:
    print line

['S25m_16Q', '-2dB']
['S25m_1_16Q', '0dB']
['S25m_2_16Q', '2dB']

Then just transpose the array,

for line in zip(*parts):
    print line

('S25m_16Q', 'S25m_1_16Q', 'S25m_2_16Q')
('-2dB', '0dB', '2dB')

edited Aug 21, 2013 at 7:34

answered Aug 21, 2013 at 7:22

will

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1 Comment

will Over a year ago

Thanks for the downvotes while editing a question i accidentally submitted before it was finished guys!

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