For numbers and matrices, we have x * y / y == x and x \ (x * y) == y. Since we have * for strings, we could implement / and \ for strings too: "abcd" / "cd" == "ab" "ab" \ "abcd" == "cd" AFAICT, it would be algebraically sound, useful, and fun. What do you think?

[image] Mason: if endswith(path, ".jl") s / ".jl" end chopsuffix(s, ".jl") was added in Julia 1.8.

[image] stevengj: chopsuffix(s, ".jl") was added in Julia 1.8. Sure, but where’s the fun in that?

[image] Mason: s[begin:end-3] Note also that this is buggy: julia> s = "α.jl" "α.jl" julia> s[begin:end-3] ERROR: StringIndexError: invalid index [2], valid nearby indices [1]=>'α', [3]=>'.' (It’s really tempting to write this, which is a key motivation for a StringIndex type .)

[image] stevengj: (It’s really tempting to write this, which is a key motivation for a StringIndex type .) Nice try, but I will not be tempted into that tar-pit of a thread today :sweat_smile:

Like the idea about regex and just checked that regular languages are closed under union, intersection, complement and concatenation. Thus one could indeed define difference as \(ra::Regex, rb::Regex) = !rb ∩ ra with ! and ∩ being complement and intersection respectively. Not sure if this efficien…

Even though regular languages are closed under complementation, unfortunately I don’t think PCRE supports complement. How do I turn any regex into an complement of itself without complex hand editing? - Stack Overflow

Was proposed and declined in Julia#13411. There’s also the consideration that / could alternatively represent path concatenation (also declined in Julia#9488).

[image] mbauman: / could alternatively represent path concatenation (also declined in Julia#9488 ). I don’t like Base./ for path concat, since it has nothing to do with division. @StefanKarpinski said in string division: "pre"\"prefix" => "fix", "suffix"/"fix" => "suf". by StefanKarpinski · P…

[image] jar1: What do you expect "de" / "cde" to do here? Good question, by analogy with integer division it should be zero, i.e., an absorbing element for *.

Just for fun: import Base: /, \ /(x::String, y::String) = begin s = replace(x, y => "", count=1); s == x ? "" : s; end \(x::String, y::String) = /(y::String, x::String) "abcd" / "cd" # == "ab" "ab" \ "abcd" # == "cd" "de" / "cde" # == ""

Left and right division for strings

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Left and right division for strings

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