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System expressions
escape/unescape : I'm not sure but wouldn't URLEncode(str) and URLDecode(str) do the job ?
toUppercase : uppercase(text)
concate (concatenate ?) : & (ie "my first string " & " and my second string" => "my frist string and my second string")
substr : mid(text, index, count)
fromCharCode : it's not in indeed, but I made this alpha WIP plugin for that, still in development, use at your own risks.
I don't believe RegEx are supported, but you can use find(src, text) to get the index of the first occurrence of "text" and then use mid() to retrieve it.
For split I believe you can use tokenat(src, index, separator). Run a loop and set the tokens as values of an array.
You can find more explicit example of use of tokenat() in the how do I FAQ IIRC.