-replace uses regex's. Capture groups are referred to by $n and not \1 as sometimes happens.
$b = $a -replace '(l+)', 'ff$1$1gg'; echo $b-replace uses regex's. Capture groups are referred to by $n and not \1 as sometimes happens.
$b = $a -replace '(l+)', 'ff$1$1gg'; echo $b