To begin with, click on the link, below :

https://community.notepad-plus-plus.org/topic/15765/faq-desk-where-to-find-regex-documentation

Cheers,

guy038

A second possibility, derived from @ekopalypse’s solution, would be :

• Open the Replace dialog ( Ctrl + H )

• SEARCH <(\w+)>.*\K\R\h*(?=</\1>)

• REPLACE Leave EMPTY

• Now, choice :

  • To tick the Wrap around option if you want to process the S/R on the whole file, from beginning to end

  • To untick the Wrap around option to process the S/R, from current location to the end of the file

  • To do a normal selection of text first and then, tick the In selection option

• Select the Regular expression search mode

• Click, exclusively on the Replace All option, whatever your choice !

Notes :

• Due to the \K syntax, inside this regex, the search process works correctly, but the “step by step” replacement, with the Replace button, is not functional :-(

• The search regex looks for a line-break, possibly followed with some blank characters ( tabulation and/or space ), ONLY IF :

  • It is preceded with <, then a name tag \w+, stored as group 1, because embedded in parentheses, then > and any subsequent character(s) .*, even 0, till the line-break

  • It is followed with the same ending tag </...>, due to the positive look-ahead structure ?=</\1>) and the \1 syntax which represents the name tag

• As the replacement zone is empty, the EOL, and the possible blank chars, are simply deleted !

Best Regards,

guy038

if your data is consistent then something like this
find what:<(\w+>)(.*)\R\h*(</\1)
replace with:\1\2\3
might do it.

So we are looking for

• a tag <(\w+>) (a less sign followed by any word followed by a greater sign
• followed by any text (.*)
• followed by a end of line char \R
• followed by horizontal spaces \h*
• followed by the start of a closing tag </ followed by what was found in the starting tag \1 -> (</\1)

<UsersC>21
</UsersC>
<UUID>be9a1528-9a/6/0/-4917-8857-12896a7693de</UUID>
<Date>2020-01-20</Date>
<UUID>7f8e38ab-ceba-45c5-ab34-834b61bad840
</UUID>

the \R denotes line endings and \h stands for horizontal spaces which can be spaces or tabs.
So this in mind you might consider
find what:<TypePost>Main\R\h+
replace with: <TypePost>Main