Text manipulation problem - how to replace text after a known value - regex

Question

I have a large text file containing filenames ending in .txt
Some of the rows of the file have unwanted text after the filename extension.
I am trying to find a way to search+replace or trim the whole file so that if a row is found with .txt, anything after this is simply removed. Example
C:\Test1.txt
C:\Test2.txtHelloWorld this is my
problem
C:\Test3.txt_____Annoying
stuff1234 .r
Desired result
C:\Test1.txt
C:\Test2.txt
C:\Test3.txt
I have tried with notepad++, or using batch/powershell, but got close, no cigar.
(Get-Content "D:\checkthese.txt") |
Foreach-Object {$_ -replace '.txt*', ".txt"} |
Set-Content "D:\CLEAN.txt"
My thinking here is if I replace anything (Wildcard*) after .txt then I would trim off what I need, but this doesnt work. I think I need to use regular expression, buy have the syntax wrong.

Answer 1

Simply change the * to a .*, like so:
(Get-Content "D:\checkthese.txt") |
Foreach-Object {$_ -replace '\.txt.*', ".txt"} |
Set-Content "D:\CLEAN.txt"
In regular expressions, * means "0 or more times", and in this case it'd act on the final t of .txt, so .txt* would only match .tx, .txt, .txtt, .txttt, etc...
., however, matches any character. This means, .* matches 0 or more of anything, which is what you want. Because of this, I also escaped the . in .txt, as it otherwise could break on filenames like: alovelytxtfile.txt, which would be trimmed to alovel.txt.
For more information, see:
Regex Tutorial - .
Regex Tutorial - *