How to [int]add in a [string]regex replacement in powershell?

PowerShell Core v6.1.0+ supports passing a script block as the replacement operand of the
-replace operator; the script block is invoked for each match found, and its output forms the replacement string, thereby enabling algorithmic replacements (rather than merely textual ones):

'foo_10.dat' -replace '(.*)_([0-9]+)(\.dat)', { 
  '{0}{1}{2}' -f $_.Groups[1].Value, 
                 ([int] $_.Groups[2].Value + 3), 
                 $_.Groups[3].Value
}

The above yields:

foo13.dat

Note how 3 was added to 10 (and the underscore was removed):

$_ inside the script block is a [System.Text.RegularExpressions.Match] instance representing the results of the match; e.g., $_.Value represents the full match, whereas $_.Groups[1].Value represents what the 1st capture group matched.

The functionality is built on the [regex] .NET type's .Replace() method, which can also used directly (but less easily) in earlier PowerShell versions - see below.

In Windows PowerShell you have two options:

• Use the -match operator first and then perform the transformation in a separate statement based on the match information reflected in the automatic $Matches variable, as suggested by kuujinbo:

  $null = 'foo_10.dat' -match '(.*)_([0-9]+)(\.dat)' # match first
  '{0}{1}{2}' -f $Matches.1, ([int] $Matches.2 + 3), $Matches.3 # build via $Matches
  

• Use the .NET framework directly, with a script block passed as a delegate (callback function) to the above-mentioned [regex]::Replace() method, as suggested by Ansgar Wiechers:

  ([regex] '(.*)_([0-9]+)(\.dat)').Replace('foo_10.dat', {
    param($match)
    '{0}{1}{2}' -f $match.Groups[1].Value, ([int] $match.Groups[2].Value + 3), $match.Groups[3].Value
  })
  

  • Note how a formal parameter - param($match) - must be declared here in order to access the match results, whereas the pure PowerShell solution at the top was able to use $_, as usual.