Researching the optimal; implementing the practical

John Cook has an interesting blog post, “Learn one Perl command”. He highlights text replacement, using the Unix command sed. But, others commented and did him one better suggesting he use a perl –pi.

PowerShell doesn’t have this out of the box. So let’s write a module.

Follow along, grab the PowerShell module here.

The PowerShell Invoke-Replace command lets you replace text:

• In place, for any or all files in the current directory, based on a file pattern match
• Using simple or regular expression matching
• Plus, it can backup the target file, to the same name with an extension you supply like .bak, before editing it

The PowerShell Module

Here are Invoke-Replace and Backup-File.

function Invoke-Replace {            
    param(            
        $Match,            
        $Replace,            
        $Include="*",            
        $BackupExtension            
    )                
            
    $files = Get-ChildItem . $Include            
    ForEach($file in $files) {            
            
      Backup-File $file $BackupExtension            
      $targetFile = $file.FullName            
                  
      [IO.File]::ReadLines($targetFile) -replace $Match, $Replace |             
        Set-Content $targetFile            
    }            
}            
            
function Backup-File {            
    param(                    
        $File,            
        $BackupExtension            
    )            
            
    if(!$BackupExtension) { return }            
            
    $path = $File.FullName            
    $destination = $path + $BackupExtension            
            
    Copy-Item -Path $path -Destination $destination            
}            
            

How to Use Invoke-Replace

In the Tests subdirectory there is TestInvokeReplace.ps1 which tries out the Invoke-Replace.

For example, the following removes commas that immediately follow a digit, for all files in the current directory that have the file extension .txt.

Invoke-Replace "(?<=\d)," "" *.txt

Here’s how to do a Backup

Adding .bak as a final parameter instructs Invoke-Replace to copy the source file to the same named file and add .bak as the extension.

Invoke-Replace "(?<=\d)," "" *.txt .bak

Using Parameter Names

Here is what Invoke-Replace looks like with the parameter names specified.

Invoke-Replace -Match "(?<=\d)," -Replace "" `
    -Include *.txt -BackupExtension .bak

Learning Perl is a big commitment

Or any other scripting or programming language for that matter. What I find interesting though, is these other more established environments have been through the mine fields and collected some terrific experiences over the decades.

Transcoding their scripting experiences often yield very usable solutions.

Check out the Script

Here is the InvokeReplace module https://github.com/dfinke/PowerShellTools. I keep several others here https://github.com/dfinke including the scripts from my book “PowerShell for Developers”.

They are open source and ready to be forked and improved.

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