Word boundary in Lucene regex
In ElasticSearch regex flavor, there is no direct equivalent to a word boundary. Initial \b is something like (^|[^A-Za-z0-9_]) if the word starts with a word char, and the trailing \b is like ($|[^A-Za-z0-9_]) if the word ends with a word char.
Thus, we need to make sure that there is a non-word char before and after word or start/end of string. Since the regex is anchored by default, all we need to make [^A-Za-z0-9_] optional at start/end of string is add .* beside and wrap with an optional grouping construct:
(.*[^A-Za-z0-9_])?word([^A-Za-z0-9_].*)?Details
(.*[^A-Za-z0-9_])?- either start of string or any 0+ chars (but a line break char, else use(.|\n)*) and then any char but a word char (basically, it is start of string followed with 1 or 0 occurrences of the pattern inside the group)word- a word([^A-Za-z0-9_].*)?- an optional sequence of any char but a word char followed with any 0+ chars, followed by the end of string position (implicit in Lucene regex).