Replace strings in a string by a substring of those strings

Question

Let's say I have a string like this:

s = '(X_xy09 and X_foobar or (X_abc123 and X_something))'

and I want to turn it into

'(xy09 and foobar or (abc123 and something))'

then - in this particular case - I could simply do

s.replace('X_', "")

which gives the desired output.

However, in my actual data there might be not only X_ but also other prefixes, so the above replace statement does not work.

What I would need instead is a replacement of

a capital letter followed by an underscore and an arbitrary sequence of letters and numbers

by

everything after the first underscore.

So, to extract the desired elements I could use:

import re
print(re.findall('[A-Z]{1}_[a-zA-Z0-9]+', s))

which prints

['X_xy09', 'X_foobar', 'X_abc123', 'X_something']

how can I now replace those elements so that I obtain

'(xy09 and foobar or (abc123 and something))'

?

@WiktorStribiżew: Would love to, but don't know how to use it in this case. If you know how, feel free to post it as an answer... :) — Cleb
– Cleb, Commented Dec 6, 2017 at 14:10
Try ideone.com/Qs9ldO. What are the exact criteria for the pattern? Should it start with a word boundary or only after (? — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Dec 6, 2017 at 14:11
@WiktorStribiżew: Ah, backreferencing! Great, that solves it. Please post it as an answer, then I upvote and accept. The pattern is as simple as described: capital letter, underscore, some arbitrary stuff. There are not always ( involved. — Cleb
– Cleb, Commented Dec 6, 2017 at 14:12

Wiktor Stribiżew · Accepted Answer · 2017-12-06 14:13:45Z

3

If you need to remove an uppercase ASCII letter with an underscore after it, only when not preceded with a word char and when followed with an alphanumeric char, you may use

import re
s = '(X_xy09 and X_foobar or (X_abc123 and X_something))'
print(re.sub(r'\b[A-Z]_([a-zA-Z0-9])', r'\1', s))

See the Python demo and a regex demo.

Pattern details

\b - a leading word boundary
[A-Z]_ - an ASCII uppercase letter and _
([a-zA-Z0-9]) - Group 1 (later referenced to with \1 from the replacement pattern): 1 alphanumeric char.

answered Dec 6, 2017 at 14:13

Wiktor Stribiżew

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1 Comment

Wiktor Stribiżew Over a year ago

Note that it is equal to re.sub(r'\b[A-Z]_(?=[a-zA-Z0-9])', '', s)

Bill the Lizard · Accepted Answer · 2017-12-06 14:25:29Z

3

If you just need to replace a capital letter followed by an underscore, you can use the regular expression r'[A-Z]_'.

s = '(X_xy09 and X_foobar or (X_abc123 and X_something))'
re.sub(r'[A-Z]_', '', s)

You may need to add to it if you have other criteria not mentioned. (For example, some of your target values follow a word boundary and some follow parentheses.) The above might give you the wrong output if you have input like XY_something. It depends on what you expect the output to be.

edited Dec 6, 2017 at 14:25

answered Dec 6, 2017 at 14:13

Bill the Lizard

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Cleb Over a year ago

Nice solution, too.

RomanPerekhrest · Accepted Answer · 2017-12-06 14:17:54Z

2

Another re.sub() approach:

import re

s = '(X_xy09 and X_foobar or (X_abc123 and X_something))'
result = re.sub(r'[A-Z]_(?=[a-zA-Z0-9]+)', '', s)

print(result)

The output:

(xy09 and foobar or (abc123 and something))

[A-Z]_(?=[a-zA-Z0-9]+) - (?=...) positive lookahead assertion, ensures that substituted [A-Z]_ substring is followed by alphanumeric sequence [a-zA-Z0-9]+

edited Dec 6, 2017 at 14:17

answered Dec 6, 2017 at 14:16

RomanPerekhrest

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2 Comments

Cleb Over a year ago

Works fine. What is the ?= part for?

RomanPerekhrest Over a year ago

@Cleb, that's positive lookahead assertion, see my explanation

Eugene Yarmash · Accepted Answer · 2017-12-06 14:19:20Z

2

You could use re.sub() with a lookahead assertion:

>>> import re
>>> s = '(X_xy09 and X_foobar or (X_abc123 and X_something))'
>>> re.sub(r'\b[A-Z]_(?=[a-zA-Z0-9])', '', s)
'(xy09 and foobar or (abc123 and something))'

from the docs:

(?=...)
Matches if ... matches next, but doesn’t consume any of the string. This is called a lookahead assertion. For example, Isaac (?=Asimov) will match 'Isaac ' only if it’s followed by 'Asimov'.

edited Dec 6, 2017 at 14:19

answered Dec 6, 2017 at 14:16

Eugene Yarmash

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1 Comment

Cleb Over a year ago

Seems to be the same as RomanPerekhrest' answer but still deserves an upvote... :)

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