Use Powershell to replace subsection of regex result

You need to group the sub-expressions you want to preserve (i.e. put them between parentheses) and then reference the groups via the variables $1 and $2 in the replacement string. Try something like this:

$regexA = '( TEST=[A-Za-z]+)14(\d\d)$'Get-ChildItem '*.txt' | ForEach-Object {    $c = (Get-Content $_.FullName) -replace $regexA, '${1}16$2' -join "`r`n"    [IO.File]::WriteAllText($_.FullName, $c)}

Here's an example using a scriptblock delegate (sometimes called an evaluator):

$regex = [regex]'( TEST=\D+)14(\d{2})\s*$'$evaluator = { '{0}16{1}' -f $args[0].Groups[1..2] }filter set-number { $regex.Replace($_, $evaluator) }foreach ($file in Get-ChildItem  "*.txt") {   ($file | get-content) | set-number | Set-Content $file.FullName }

It's arguably more complex than the -replace operator, but lets you use powershell operators to construct the replacement text, so you can do anything you can put in a script block.

Try this:

Get-ChildItem  "*.txt" |Foreach-Object {  $c = $_ | Get-Content | Foreach {$_ -replace '(?<=TEST=\D+)14(?=\d{2}(\D+|$))','16'}  $c | Out-File $_.FullName -Enc Ascii}