Using regex to match string between two strings while excluding strings

Question

Following on from a previous question in which I asked:

How can I use a regular expression to match text that is between two strings, where those two

Answer 1

Tola, resurrecting this question because it had a fairly simple regex solution that wasn't mentioned. This problem is a classic case of the technique explained in this question to "regex-match a pattern, excluding..."

The idea is to build an alternation (a series of |) where the left sides match what we don't want in order to get it out of the way... then the last side of the | matches what we do want, and captures it to Group 1. If Group 1 is set, you retrieve it and you have a match.

So what do we not want?

First, we want to eliminate the whole outer block if there is unwanted between outer-start and inner-start. You can do it with:

outer-start(?:(?!inner-start).)*?unwanted.*?outer-end

This will be to the left of the first |. It matches a whole outer block.

Second, we want to eliminate the whole outer block if there is unwanted between inner-end and outer-end. You can do it with:

outer-start(?:(?!outer-end).)*?inner-end(?:(?!outer-end).)*?unwanted.*?outer-end

This will be the middle |. It looks a bit complicated because we want to make sure that the "lazy" *? does not jump over the end of a block into a different block.

Third, we match and capture what we want. This is:

inner-start\s*(text-that-i-want)\s*inner-end

So the whole regex, in free-spacing mode, is:

(?xs)
outer-start(?:(?!inner-start).)*?unwanted.*?outer-end # dont want this
| # OR (also don't want that)
outer-start(?:(?!outer-end).)*?inner-end(?:(?!outer-end).)*?unwanted.*?outer-end
| # OR capture what we want
inner-start\s*(text-that-i-want)\s*inner-end

On this demo, look at the Group 1 captures on the right: It contains what we want, and only for the right block.

In Perl and PCRE (used for instance in PHP), you don't even have to look at Group 1: you can force the regex to skip the two blocks we don't want. The regex becomes:

(?xs)
(?: # non-capture group: the things we don't want
outer-start(?:(?!inner-start).)*?unwanted.*?outer-end # dont want this
| # OR (also don't want that)
outer-start(?:(?!outer-end).)*?inner-end(?:(?!outer-end).)*?unwanted.*?outer-end
)
(*SKIP)(*F) # we don't want this, so fail and skip
| # OR capture what we want
inner-start\s*\Ktext-that-i-want(?=\s*inner-end)

See demo: it directly matches what you want.

The technique is explained in full detail in the question and article below.

Reference

• How to match (or replace) a pattern except in situations s1, s2, s3...
• Article about matching a pattern unless...