I’m to find a regular expression to match strings inside single but not multiple particular characters. For example:

this is =one= and =two= but not ==three== or ==four== etc

Here I’m using = as a delimiter, and I want to match the one and two because they’re inside single delimiters but not three or four because they’re inside multiple delimiters.

I thought using negative lookahead and lookbehind would do the trick:

/(?

but I end up matching the other strings as well, so I obviously don’t understand properly hot negative lookahead and lookbehind work. I thought the expression means = not preceded by a =, and so on.

A bigger test includes multiline strings:

this is =one= and =two= but not ==three== or ==four==.

====
this shouldn’t count at all
====

There might be another approach, which would be welcome, but one using lookahead and lookbehind would be more educational I think.

I’m using PHP and I plan to use the preg_replace_callback() function.