regex/python to find and replace specific number within string

Question

I need to replace numbers (corners) that occur within a longer string that all look similar to this:

[ 17 plane_17 \ 23 25 17 99 150 248 \ noname ]

My function takes an "old" number to be replaced with a "new" number, and e.g. if that old number is 17 and the new is 19, then the outcome should be:

[ 17 plane_17 \ 23 25 19 99 150 248 \ noname ]

Note that only the numbers within \ \ should be replaced (these could also be / / ).

To do this I tried to set up a regex substitution with the intention of avoiding numbers outside of \ \ or / /: newplane = re.compile(r"[^[_] (" + str(oldcorner) + ")").sub(str(newcorner), oldplane)

I quickly realised that this doesn't work since regex searches from the start of the line and then fails if it doesn't match the pattern.

There must be some clever way of doing it still that I don't know about.. Any suggestions?

will they be between '\\', or single '\'? My first instinct would be to split it at '\' (or double), call plain old replace( old, new ) on every second element of the result and then join them back together at '\'. Not very pythonic though, interested to see if someone has a neat way — max k.
– max k., Commented Mar 11, 2013 at 14:20
They'll all be either within single '\' or single '/'. That's not a bad idea! Would remove the regex too.... — jodles
– jodles, Commented Mar 11, 2013 at 14:23
yeah would keep things simple, not sure how you would handle both '\' and '/' in the same line though — max k.
– max k., Commented Mar 11, 2013 at 14:25

vpekar · Accepted Answer · 2013-03-11 14:30:19Z

4

You can use a callback function inside the sub part of the regex:

import re

def callback(match):
    return match.group(0).replace('17', '19')

s = "[ 17 plane_17 \ 23 25 17 99 150 248 \ noname ]"

s = re.compile(r'\\.+?\\').sub(callback, s)

print s

Prints:

[ 17 plane_17 \ 23 25 19 99 150 248 \ noname ]

answered Mar 11, 2013 at 14:30

vpekar

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6 Comments

max k. Over a year ago

Where did the slashes get to?

jodles Over a year ago

Thanks for this, vpekar! Really clever with a callback function!

asermax Over a year ago

One issue with this: if there's a three digits (or more) number with the '17' as part of it, the callback will also replace it.

Timo Over a year ago

Callback means that first the callback runs and then sub? callback gets parameter s?

Timo Over a year ago

Does the question mark (non-greedy operator) make sense because there is no decision between greedy and non-greedy?

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asermax · Accepted Answer · 2013-03-11 14:36:26Z

1

In addition to vpekar answer, you can also use backreferences of your pattern on the replacement string, so you can try to match all between / or \ and recreate the string using your new number and backreferences:

line = '[ 17 plane_17 \ 23 25 17 99 150 248 \ noname ]'
re.sub(r'([\\|/].*\s)(?:17)(\s.*[\\|/])', r'\g<1>19\2', line)

which returns:

'[ 17 plane_17 \ 23 25 19 99 150 248 \ noname ]'

answered Mar 11, 2013 at 14:36

asermax

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3 Comments

jodles Over a year ago

Thanks asermax! I really like this one-liner solution too! There are clearly some gaps in my regex knowledge. Thanks!

Timo Over a year ago

Can you explain parts of it. sub has 3 parameters: old, new, string. You could leave the first r. Where is backreference no.1? no.2 is \2.

Timo Over a year ago

A non capturing group is not needed, simpler: re.sub(r'([\\|/].*\s)(17)(\s.*[\\|/])', r'\g<1>19\3', line)

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