How to handle word in the middle using "look ahead" and "look behind" in Regex?

Question

From answer of @skyfoot at this question regex-lookahead-lookbehind-and-atomic-groups

He said that:

given the string `foobarbarfoo`

    bar(?=bar)     finds the first bar.
    bar(?!bar)     finds the second bar.
    (?<=foo)bar    finds the first bar.
    (?<!foo)bar    finds the second bar.

you can also combine them

    (?<=foo)bar(?=bar)    finds the first bar.

What's happened suppose i have string "barfoobarbarfoo"

And i want to find these bold text

"barfoobarbarfoo"

The regex might be: (?<=bar)foo(????)bar(?=foo)

The question is that what expression should be in the middle (look ahead or look behide)?

Answer 1

try this pattern

foo(?=bar)|(?<=bar)bar  

Demo

Answer 2

To complete your example:

bar(?=bar)     finds the `bar` followed by `bar`
bar(?!bar)     finds the `bar` NOT followed by `bar`
(?<=foo)bar    finds the `bar` preceeded by `foo`
(?<!foo)bar    finds the `bar` NOT preceeded by `foo`

So, in your case, you want the foo preceeded by bar AND followed by bar. Using the example above, this should give:

(?<=bar)foo(?=bar)

Then, to find the bar preceeded by bar AND followed by foo you should use:

(?<=bar)bar(?=foo)

Finally, to combine those two patterns, you should use the OR operator and you end up with the following:

(?<=bar)foo(?=bar)|(?<=bar)bar(?=foo)

DEMO