Replacing values that fail a match test using Python's regex library

Question

I have a very large string consiting of a series of numbers separated by one or more spaces. Some of the numbers are equal to -123, and the rest can be any random number.

example_string = "102.3  42.89  98  812.7  374  5  -123  8  -123  13  -123  21..."

I would like to replace the values that are not equal to -123 with 456 in the most efficient way possible.

updated_example_string = "456  456  456  456  456  456  -123  456  -123  456  -123  456..."

I know that python's regular expression library has a sub method that will replace matching values quite efficiently. Is there a way to replace values that DO NOT match? As I mentioned, this is a rather large string, coming from a source file around 100MB. Assuming there's a way to use re.sub to accomplish this task, is that even the correct/most efficient way of handling such problem?

Possible duplicate of Python regular expression to replace everything but specific words — jberrio
– jberrio, Commented Jan 11, 2019 at 5:51

Nick · Accepted Answer · 2019-01-11 06:00:08Z

2

You can use this regex:

(^|\s)(?!-123(\s|$))-?[0-9.]+(?=\s|$)

It looks for the start of string or a space, not followed by -123 and space of end of string (using a negative lookahead) then some number of digits or a ., followed by either a space or end of string.

Then you can replace with \g<1>456 to turn all those numbers into 456. The \g<1> in the replacement preserves any space captured by the first group.

Demo on regex101

In Python:

import re
string = "102.3  42.89 -1234 98  -812.7  374  5  -123  8  -123  13  -123  21 -123"
print re.sub(r'(^|\s)(?!-123(\s|$))-?[0-9.]+(?=\s|$)', '\g<1>456', string)

Output

456  456 456 456  456  456  456  -123  456  -123  456  -123  456 -123

Demo on rextester

answered Jan 11, 2019 at 6:00

Nick

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Comments

The fourth bird · Accepted Answer · 2019-01-11 15:52:33Z

You could match only the numbers between whitspace boundaries and the use re.sub with a callback function to check if the match is not -123. If it not, relace it with 456

(?<!\S)-?\d+(?:\.\d+)?(?!\S)

Explanation

(?<!\S) Negative lookbehind to assert what is on the left is not a non-whitespace character
-? Optional -
\d+(?:\.\d+)? Match 1+ digits with an optional part that matches a . and 1+ digits
(?!\S) Negative lookahead to assert what is on the right is not a non-whitespace character

Example

import re
pattern = r"(?<!\S)-?\d+(?:\.\d+)?(?!\S)"
s = "102.3  42.89  98  812.7  374  5  -123  8  -123  13  -123  21"

print(re.sub(pattern, lambda m: "456" if m.group() != "-123" else m.group(), s))

Result

456  456  456  456  456  456  -123  456  -123  456  -123  456

See the Regex demo | Python demo

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