You’re welcome, @Inshita-Rawat. If you follow the links in the FAQ, especially to one of the try-it-live websites, like regexr.com (*), they will explain the individual elements of the regular expression in great detail.
*: I usually use regexr, and switch it from “JAVASCRIPT” to “PCRE” in the upper right. PCRE comes come close to Notepad++'s Boost-based regular expressions — since both Boost↗ and PCRE↗ are based on Perl regular expressions, though there are differences (both from each other, and from Perl).
If you paste my ^(.*?) (\d.*) regular expression into regexr.com with PCRE, and FLAGS=global,multiline (/gm), along with example data – see https://regexr.com/466oe – you can see the explanation, or by clicking on REPLACE, you can input my replace string, and see what it will become.