regex - Regular expressions: Ensuring b doesn't come between a and c

Question

Here's something I'm trying to do with regular expressions, and I can't figure out how. I have a big file, and strings abc, 123 and xyz that appear multiple times throughout the file.

I want a regular expression to match a substring of the big file that begins with abc, contains 123 somewhere in the middle, ends with xyz, and there are no other instances of abc or xyz in the substring besides the start and the end.

Is this possible with regular expressions?

Answer 1

When your left- and right-hand delimiters are single characters, it can be easily solved with negated character classes. So, if your match is between a and c and should not contain b (literally), you may use (demo)

a[^abc]*c

This is the same technique you use when you want to make sure there is a b in between the closest a and c (demo):

a[^abc]*b[^ac]*c

When your left- and right-hand delimiters are multi-character strings, you need a tempered greedy token:

abc(?:(?!abc|xyz|123).)*123(?:(?!abc|xyz).)*xyz

See the regex demo

To make sure it matches across lines, use re.DOTALL flag when compiling the regex.

Note that to achieve a better performance with such a heavy pattern, you should consider unrolling it. It can be done with negated character classes and negative lookaheads.

Pattern details:

• abc - match abc
• (?:(?!abc|xyz|123).)* - match any character that is not the starting point for a abc, xyz or 123 character sequences
• 123 - a literal string 123
• (?:(?!abc|xyz).)* - any character that is not the starting point for a abc or xyz character sequences
• xyz - a trailing substring xyz

See the diagram below (if re.S is used, . will mean AnyChar):

See the Python demo:

import re
p = re.compile(r'abc(?:(?!abc|xyz|123).)*123(?:(?!abc|xyz).)*xyz', re.DOTALL)
s = "abc 123 xyz
abc abc 123 xyz
abc text 123 xyz
abc text xyz xyz"
print(p.findall(s))
// => ['abc 123 xyz', 'abc 123 xyz', 'abc text 123 xyz']