Conditionally replace string in file PowerShell

Regex to the rescue!

"(computer)(?!\.contoso\.com)"

You want to match on "computer" name, and use negative look ahead to ignore ones that already have the FQDN portion.

$test = Get-Content C:\temp\test.txt
$test -replace "($compname)(?!\.contoso\.com)", $fqdn

The first parameter is a regular expression, so you want to match on $compname that does not occur before the domain portion of the FQDN. You can achieve this with a Negative Lookahead:

$domain = [regex]::Escape( $fqdn.split( '.', 2 )[1] )
$test -replace "${compname}(?!${domain})", $fqdn

Let's break down what this does:

• $domain = [regex]::Escape( $fqdn.split( '.', 2 )[1] ) - Extract the domain portion of the FQDN, and make sure it's escaped so no special characters are interpreted as regex syntax
• "${compname}(?!\.${domain})" - Regex pattern to match on - let's break this down
  • ${compname} - Insert the $compname variable as part of the string
  • \. - Match on a literal .; the backslash is required as . is a special token (matches any character) when processing regular expressions
  • () - Grouped expression - let's also break this down
    • ?! - Negative lookahead operator, tells the grouped expression that this group cannot match after the preceeding expression. If matched, this group is discarded from the result.
    • ${domain} - Same as ${compname}, just inserting the variable into the expression

I figured out a way to overwrite the entire line, rather than replace comp name.

$match = $test | select-string -pattern "serverName"
$test -replace $match,"serverName = $fqdn" | set-content c:\temp\test.txt

This guarantees whatever the entry is will be overwritten by the FQDN.