ios - RegEx negative-lookahead and behind to find characters not embedded within a wrapper -

i match strings/characters not surrounded well-defined string-wrapper. in case wrapper '@l@' on left of string , '@r@' on right of string.

with following string example:

this @l@string@r@ , it's @l@good or ok@r@ change characters in next string 

i able search (any number of characters) change them on case case basis. example:

1. searching "in", match twice - word 'in', , 'in' contained within last word 'string'.
2. searching "g", should found within word 'change' , in final word string (but not first occurrence of string contained within wrapper).

i'm familiar how lookahead works in sense identifies match, , doesn't return matching criteria part of identified match. unfortunately, can't head around how it.

i've been playing @ http://regexpal.com/ can't seem find works. examples i've found ios problematic, perhaps javascript tester tiny bit different.

i took guidance previous question asked, seemed same sufficiently different mean couldn't work out how reuse it:

replacing 'non-tagged' content in web page

any ideas?

at first @l@ @r@ blocks , use alternation operator | match string in remaining string. differentiate matches, put in inside capturing group.

@l@.*?@r@|(in) 

demo

or

use negative lookahead assertion. match sub-string in if it's not followed @l@ or @r@, 0 or more times , further followed @r@. match in's not present inside @l@ , @r@ blocks.

in(?!(?:(?!@[rl]@).)*@r@) 

demo