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

Answer 1

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

(?<!x)abc

In a replacement:

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

Answer 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

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