Extracting pattern in a text file

I have a file that has following data. How to extract only the letters till ‘_’ in each line?

Xy678_dkashdkahd
Xy_dkashdkahd
Xy678sga_dkashdkahd
9999_dkashdkahd

Hmm. Probably a lot of approaches.

$lines = get-content file.txt
foreach ($line in $lines) {
 $pieces = $line -split '_'
 write-output $pieces[0]
}

Might be one approach.

Great! Thank you Sir! I was banging my head to get this in a single line using PATTERN/MATCH etc. But your solution works. Thank you much.

If you wat a single line solution:
(get-content file.txt) -replace ‘^([^]+).+’,’$1’

Thank you Rob! This is what I was looking for at the first place. I know some regex. So I could actually understand it. Thank you both.