Fix printing of variable names in IJuliaMode (#2423)

* Fix printing of variable names in IJuliaMode

* Tweak the regex slightly

src/print.jl

@@ -321,12 +321,23 @@ function function_string(::Type{REPLMode}, v::AbstractVariableRef)
 end
 function function_string(::Type{IJuliaMode}, v::AbstractVariableRef)
     var_name = name(v)
-    if !isempty(var_name)
-        # TODO: This is wrong if variable name constains extra "]"
-        return replace(replace(var_name, "[" => "_{", count = 1), "]" => "}")
-    else
+    if isempty(var_name)
         return "noname"
     end
+    # We need to escape latex math characters that appear in the name.
+    # However, it's probably impractical to catch everything, so let's just
+    # escape the common ones:
+    # Escape underscores to prevent them being treated as subscript markers.
+    var_name = replace(var_name, "_" => "\\_")
+    # Escape carets to prevent them being treated as superscript markers.
+    var_name = replace(var_name, "^" => "\\^")
+    # Convert any x[args] to x_{args} so that indices on x print as subscripts.
+    m = match(r"^(.*)\[(.+)\]$", var_name)
+    if m !== nothing
+        var_name = m[1] * "_{" * m[2] * "}"
+    end
+
+    return var_name
 end
 
 #------------------------------------------------------------------------

test/print.jl

@@ -326,7 +326,7 @@ function printing_test(ModelType::Type{<:JuMP.AbstractModel})
         io_test(REPLMode, y[2], "bar[2]")
         io_test(IJuliaMode, y[2], "bar_{2}")
         io_test(REPLMode, z[:red], "color_123[red]")
-        io_test(IJuliaMode, z[:red], "color_123_{red}")
+        io_test(IJuliaMode, z[:red], "color\\_123_{red}")
         io_test(REPLMode, v[2, 1], "i123123[1,2]")
         io_test(IJuliaMode, v[2, 1], "i123123_{1,2}")
         io_test(REPLMode, w[1, 3], "symm[1,3]")
@@ -788,3 +788,14 @@ end
     printing_test(JuMPExtension.MyModel)
     model_extension_printing_test(JuMPExtension.MyModel)
 end
+
+@testset "Print IJulia with math operators" begin
+    model = Model()
+    @variable(model, x_ab)
+    io_test(IJuliaMode, x_ab, "x\\_ab")
+    @variable(model, y[1:2], base_name = "y[:a]")
+    io_test(IJuliaMode, y[1], "y[:a]_{1}")
+    io_test(IJuliaMode, y[2], "y[:a]_{2}")
+    @variable(model, z, base_name = "z^1")
+    io_test(IJuliaMode, z, "z\\^1")
+end