@sani-bani
Try this:
Invoke Replace dialog (default key: ctrl+h)
Find what zone: (?-s)^(.+)\R(.+)(?:\R|\z)
Replace with zone: \1:\2
Wrap around checkbox: ticked or unticked, as you like
Search mode selection: Regular expression
Action: Press Replace or Replace All button, as you like
Here’s how it works:
THE FIND EXPRESSION:
(?-s)^(.+)\R(.+)(?:\R|\z)
[Use these options for the whole regular expression][
1 ] (?-s)
[(hyphen inverts the meaning of the letters that follow)][
1 ] -
[Dot doesn’t match line breaks][
1 ] s
[Assert position at the beginning of a line (at beginning of the string or after a line break character) (carriage return and line feed, form feed)][
2 ] ^
[Match the regex below and capture its match into backreference number 1][
3 ] (.+)
[Match any single character that is NOT a line break character (line feed, carriage return, form feed)][
4 ] .+
[Between one and unlimited times, as many times as possible, giving back as needed (greedy)][
5 ] +
[Match a line break (carriage return and line feed pair, sole line feed, sole carriage return, vertical tab, form feed)][
6 ] \R
[Match the regex below and capture its match into backreference number 2][
3 ] (.+)
[Match any single character that is NOT a line break character (line feed, carriage return, form feed)][
4 ] .+
[Between one and unlimited times, as many times as possible, giving back as needed (greedy)][
5 ] +
[Match the regular expression below][
3 ] (?:\R|\z)
[Match this alternative (attempting the next alternative only if this one fails)][
7 ] \R
[Match a line break (carriage return and line feed pair, sole line feed, sole carriage return, vertical tab, form feed)][
6 ] \R
[Or match this alternative (the entire group fails if this one fails to match)][
7 ] \z
[Assert position at the very end of the string][
8 ] \z
THE REPLACE EXPRESSION:
\1:\2
[Insert the text that was last matched by capturing group number 1][
9 ] \1
[Insert a colon][
10 ] :
[Insert the text that was last matched by capturing group number 2][
9 ] \2
Created with RegexBuddy
[1 ]: https://www.regular-expressions.info/modifiers.html
[2 ]: https://www.regular-expressions.info/anchors.html
[3 ]: https://www.regular-expressions.info/brackets.html
[4 ]: https://www.regular-expressions.info/dot.html
[5 ]: https://www.regular-expressions.info/repeat.html
[6 ]: https://www.regular-expressions.info/nonprint.html
[7 ]: https://www.regular-expressions.info/alternation.html
[8 ]: https://www.regular-expressions.info/anchors.html#az
[9 ]: https://www.regular-expressions.info/replacebackref.html
[10 ]: https://www.regular-expressions.info/characters.html
RegexBuddy settings to emulate N++ regex engine: Application=boost::regex 1.54-1.57 / flavor=Default flavor / replacement flavor=All flavor / ^$ match at line breaks / Numbered capture / Allow zero-length matches