Lets see with an example :

We want to search a string “final” which must be preceded by “semi“. Here is two strings below:
final
semifinal

Regex will be :

(?<=semi)final

Output :
final
semifinal


Regex Explanation :

(?<= : Starting of Positive Lookbehind.
semi : String that should be preceded by.
) : Ending of Positive Lookbehind.
final : Actual string to match.