PowerShell-Technique: PowerShell: Getting all String occurrence Indices

Recently, I participated in AdventOfCode 2023 and encountered challenges that required finding the precise location of a character in a string, along with all its occurrences. Take, for instance, the string 'Hello, world!'. I needed to identify all positions of the character 'l': 2, 3, and 10.

The typical method, .IndexOf('l'), provides only the first occurrence (in this case, 2). To obtain all positions, I devised a PowerShell function that iterates through the string using the IndexOf() method with a position parameter. Here's the function:

function Get-IndexOfAll {
    param (
        [string]$String,
        [string]$SearchString
    )
    $IndexList = [System.Collections.Generic.List[int]]::new()
    $Index = $String.IndexOf($SearchString)
    while($Index -ne -1){
        $IndexList.Add($Index)
        $Index = $String.IndexOf($SearchString,$Index+1)
    }
    return $IndexList
}

Additionally, I demonstrated in a previous post how to extend the TypeData of string objects using Update-TypeData:

$IndexOfAll = {
    param(
        $SearchString
    )
    $IndexList = [System.Collections.Generic.List[int]]::new()
    $Index = $this.IndexOf($SearchString)
    while($Index -ne -1){
        $IndexList.Add($Index)
        $Index = $this.IndexOf($SearchString,$Index+1)
    }
return $indexlist
}

$etd = @{
    TypeName = 'string'
    MemberType = 'Scriptmethod'
    MemberName = 'IndexOfAll'
    Value = $IndexOfAll
}
Update-TypeData @etd

Now, you can easily find all occurrences of a character in a string using the IndexOfAll method.

If you have any thoughts or feedback on this topic, feel free to share them with me on Twitter at Christian Ritter.

Best regards, Christian