#PSTip Replacing special characters

Once upon a time I answered Stack Overflow question about easy way to replace ‘special’ characters with something ‘web safe’. I answered question with the following code:

$Replacer = @{
    Å = 'aa'
    é = 'e'
}

$string_to_fix = 'æøåéüÅ'
$pattern = "[$(-join $Replacer.Keys)]"
[regex]::Replace(
    $string_to_fix, 
    $pattern, 
    { 
        $Replacer[$args[0].value] 
    }
)

My answer was accepted by the person asking question, but I was not fully satisfied with it. It maybe worked fine for him, but when I tried to apply the same pattern to text with Polish national characters I couldn’t get what I wanted. My problem was the fact that hash tables in PowerShell are case-insensitive. To cover both upper and lower case characters I would have to do it in two steps:

$lower = @{
    ą = 'a'
    ć = 'c'
    ę = 'e'
    ł = 'l'
    ń = 'n'
    ó = 'o'
    ś = 's'
    ż = 'z'
    ź = 'z'
}

$patternLower = "[$(-join $lower.Keys)]"
$lowerReplaced = [regex]::Replace(
    'Zażółć GĘŚLĄ jaźń',
    $patternLower,
    {
        $lower[$args[0].value]
    }
)

$patternUpper = $patternLower.ToUpper()
[regex]::Replace(
    $lowerReplaced,
    $patternUpper,
    {
        ($lower[$args[0].value]).ToUpper()
    }
)

Zazolc GESLA jazn

In the first step I use $lower hash table and match any lower-case Polish character. I save result to $lowerReplace variable and use it in the next Replace() call. In the second step I match upper-case Polish characters. Hash table is case-insensitive so it will return lower-case replacement. All I need to do is to convert it ToUpper().

I would prefer to do it in one step, with hash table (or any other dictionary) that is case-sensitive. I didn’t have time to investigate it further back than. But few weeks ago I came across solution for my problem. Even better – it was used in a tip that had exactly same purpose! It was in PowerShell.com tip about converting special characters..

With information and code provided in that tip I was able to build case-sensitive hash table. But instead of listing all letters one by one I decided to take it step further and build hash table slightly different:

$Replacer = New-Object hashtable
foreach ($letter in 
    Write-Output ą a Ą A ć c Ć C ę e Ę E ł l Ł L ń n Ń N ó o Ó O ś s Ś S ż z Ż Z ź z Ź Z) {
    $foreach.MoveNext() | Out-Null
    $Replacer.$letter = $foreach.Current
}

First, I create simple array by using the Write-Output cmdlet. Each item that I want to replace is followed by a string that should be used as a replacement string. This array is processed by foreach() loop. Inside this loop I use $foreach automatic variable. With $foreach.MoveNext() method and $foreach.Current property I can access two elements in a single cycle. Note that $letter is not updated when MoveNext() method is used.

Once our case-sensitive hash table is defined we can use it in Replace() method:

$pattern = "[$(-join $Replacer.Keys)]"
[regex]::Replace(
    'Zażółć GĘŚLĄ jaźń',
    $pattern,
    {
        $Replacer[$args[0].value]
    }
)

Zazolc GESLA jazn

About the author: Bartek Bielawski

Bartek is a busy IT Admin working for an international company, Optiver. He loves PowerShell and automation. That love got him the honors of a Microsoft MVP. He shares his knowledge on his blog. You can also find him on Twitter: @bielawb.

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