正则表达式RE：全部但不是这种模式

Question

quick question... I need a regex that matches a particular letter in a code unless it is contained in a certain pattern.

I want something that matches N followed or preceded by anything aslong as it isn't preceded IMMEDIATELY by C(=O).

Example:

C(=O)N

Should not match

C(=O)CN

Should match

But it doesn't need an anchor because:

C(=O)NCCCN

Should match because of the N at the end

So far i have this:

(?!C\(=O\)N$)[N]

Any help would be appreciated.

Answer 1

You can use a negative lookbehind:

(?<!C\(=O\))N

See the regex demo

The N will get matched only when not preceded immediately with a literal C(=O) sequence.

The (?<!...) is called a negative lookahead . It does not consume characters (does not move the regex index), but just checks if something is absent from the string before the current position. If the text is matched, the match is failed (there is no match). See Lookarounds for more details.

In Python : r'(?<!C\\(=O\\))N' :

import re
p = re.compile(r'(?<!C\(=O\))N')
strs = ["C(=O)N", "C(=O)CN", "C(=O)NCCCN"]
print([x for x in strs if p.search(x)])

Answer 2

Use a negative look-behind instead:

(?<!C\(=O\))N

See this regex101 example .

Regards.