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 NO

Answer 1

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

• Lookahead and Lookbehind Zero-Length Assertions
• Mastering Lookahead and Lookbehind

Answer 2

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

(?<!x)abc

In a replacement:

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