Fix regex delimiter detection in syntax highlighter (#16394)

Author: HertzDevil

spec/std/crystal/syntax_highlighter/colorize_spec.cr

@@ -127,6 +127,10 @@ describe Crystal::SyntaxHighlighter::Colorize do
 
     it_highlights "Set{1, 2, 3}", %(\e[36mSet\e[39m{\e[35m1\e[39m, \e[35m2\e[39m, \e[35m3\e[39m})
 
+    it_highlights "foo(/Name: /)", %(foo(\e[93m/Name: /\e[39m))
+    it_highlights "foo[/Name: /]", %(foo[\e[93m/Name: /\e[39m])
+    it_highlights "Foo{/Name: /}", %(\e[36mFoo\e[39m{\e[93m/Name: /\e[39m})
+
     it_highlights <<-CRYSTAL, <<-ANSI
       foo, bar = <<-FOO, <<-BAR
         foo

spec/std/crystal/syntax_highlighter/html_spec.cr

@@ -122,6 +122,10 @@ describe Crystal::SyntaxHighlighter::HTML do
 
     it_highlights "Set{1, 2, 3}", %(<span class="t">Set</span>{<span class="n">1</span>, <span class="n">2</span>, <span class="n">3</span>})
 
+    it_highlights "foo(/Name: /)", %(foo(<span class="s">/Name: /</span>))
+    it_highlights "foo[/Name: /]", %(foo[<span class="s">/Name: /</span>])
+    it_highlights "Foo{/Name: /}", %(<span class="t">Foo</span>{<span class="s">/Name: /</span>})
+
     it_highlights <<-CRYSTAL, <<-HTML
     foo, bar = <<-FOO, <<-BAR
       foo

src/crystal/syntax_highlighter.cr

@@ -70,11 +70,16 @@ abstract class Crystal::SyntaxHighlighter
   end
 
   private def slash_is_not_regex(last_token_type type, space_before)
-    return nil if type.nil?
-
-    type.number? || type.const? || type.instance_var? ||
-      type.class_var? || type.op_rparen? ||
-      type.op_rsquare? || type.op_rcurly? || !space_before
+    case type
+    when Nil
+      false
+    when .number?, .const?, .instance_var?, .class_var?, .op_rparen?, .op_rsquare?, .op_rcurly?
+      true
+    when .op_lparen?, .op_lsquare?, .op_lcurly?
+      false
+    else
+      !space_before
+    end
   end
 
   private def highlight_normal_state(lexer, break_on_rcurly = false)