replace-substring.vim

function! ReplaceSubstring(str, from, to)
    let idx = stridx(a:str, a:from)
    if idx == -1
        return a:str
    endif
    let before = idx == 0 ? '' : a:str[: idx-1]
    let after  = a:str[idx + strlen(a:from) :]
    return before . a:to . after
endfunction


" tests
SimpleTapSingleTest

Is ReplaceSubstring("foobarbaz", "foo", "hi"), "hibarbaz"
Is ReplaceSubstring("foobarbaz", "oo", "hi"), "fhibarbaz"
Is ReplaceSubstring("foobarbaz", "bar", "hi"), "foohibaz"
Is ReplaceSubstring("foobarbaz", "baz", "hi"), "foobarhi"
Is ReplaceSubstring("fooBarbaz", "ba", "hi"), "fooBarhiz"

Is ReplaceSubstring("", "foo", "hi"), ""
Is ReplaceSubstring('&\;?[a-z]%$"', "[a-z]", "foo"), '&\;?foo%$"'

Done