How do I "split" a string in PowerShell using a string of multiple characters?

There is no need to create a function.

You can use the split function in two different ways. If you use:

"One two three keyword four five".Split("keyword")

every single character inside the brackets is used as splitter. But if you instead use:

"One two three keyword four five" -Split ("keyword")

the string "keyword" is used as splitter.

EDIT 2020-05-23: I've moved my code to GitHub, here, where I've made updates to cover a few edge cases: https://github.com/franklesniak/PowerShell_Resources/blob/master/Split-StringOnLiteralString.ps1

The -split operator expects RegEx, but can be made to do this well. However, the -split operator is only available on Windows PowerShell v3+, so it does not fit the requirements in the question.

A [regex] object has a Split() method that can handle this as well, but it expects RegEx as the "splitter". To get around this, we can use a second [regex] object and call the Escape() method to convert our literal string "splitter" into escaped RegEx.

Wrapping all this up into an easy to use function that works back to PowerShell v1 and also works on PowerShell Core v6.

function Split-StringOnLiteralString{    trap    {        Write-Error "An error occurred using the Split-StringOnLiteralString function. This was most likely caused by the arguments supplied not being strings"    }    if ($args.Length -ne 2) `    {        Write-Error "Split-StringOnLiteralString was called without supplying two arguments. The first argument should be the string to be split, and the second should be the string or character on which to split the string."    } `    else `    {        if (($args[0]).GetType().Name -ne "String") `        {            Write-Warning "The first argument supplied to Split-StringOnLiteralString was not a string. It will be attempted to be converted to a string. To avoid this warning, cast arguments to a string before calling Split-StringOnLiteralString."            $strToSplit = [string]$args[0]        } `        else `        {            $strToSplit = $args[0]        }        if ((($args[1]).GetType().Name -ne "String") -and (($args[1]).GetType().Name -ne "Char")) `        {            Write-Warning "The second argument supplied to Split-StringOnLiteralString was not a string. It will be attempted to be converted to a string. To avoid this warning, cast arguments to a string before calling Split-StringOnLiteralString."            $strSplitter = [string]$args[1]        } `        elseif (($args[1]).GetType().Name -eq "Char") `        {            $strSplitter = [string]$args[1]        } `        else `        {            $strSplitter = $args[1]        }        $strSplitterInRegEx = [regex]::Escape($strSplitter)        [regex]::Split($strToSplit, $strSplitterInRegEx)    }}

Now, using the earlier example:

PS C:\Users\username> Split-StringOnLiteralString "One two three keyword four five" "keyword"One two three four five

Volla!