Python regex: substitute all occurrence of a certain string NOT after a specific character

Question

How can I substitute all occurrence of a certain string NOT after a specific character in Python?

For example, I want to substitute all occurrence of abc NOT with a x before them with def. Here is the code:

re.sub(r'(^|[^x])abc', r'\1def', string)

If the string doesn't have consecutive abcs, the code works perfectly. However, if I have string='abcabc', the code won't work. Is there any way to solve this problem?

Casimir et Hippolyte · Accepted Answer · 2014-07-03 22:47:45Z

3

With a negative lookbehind assertion (?<!...) (i.e. not preceded by):

(?<!x)abc

In a replacement:

re.sub(r'(?<!x)abc', r'def', string)

answered Jul 3, 2014 at 22:47

Casimir et Hippolyte

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zx81 · Accepted Answer · 2014-07-03 22:48:47Z

2

result = re.sub("(?<!x)abc", "def", subject)

The negative lookbehind (?<!x) asserts that what precedes is not x
abc matches abc
We replace with def

Reference

answered Jul 3, 2014 at 22:48

zx81

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