regex - Remove lines from file if do not match regular expression -

for every file in directory wish remove lines match regular expression (beginning |b example) using powershell.

i think can via get-childitem on directory, foreach-object, get-content , sort of if -match i'm struggling fit together.

any massively appreciated. first time i've ever written powershell script.

something below should in right direction

$files = get-childitem "c:\your\dir"  foreach ($file in $files) {   $c = get-content $file.fullname | { $_ -notmatch "^\|b" }   $c | set-content $file.fullname }