Fix Dollar-Quoted Strings (postgres + cql)

This commit refactors code from the markdown lexer into the chroma
package, and alters the PostgreSQL and CQL lexers to make use of it.

Additionally, an example markdown with the various sublexers is added.

regexp.go

@@ -33,7 +33,7 @@ func (e EmitterFunc) Emit(groups []string, lexer Lexer) Iterator { return e(grou
 func ByGroups(emitters ...Emitter) Emitter {
 	return EmitterFunc(func(groups []string, lexer Lexer) Iterator {
 		iterators := make([]Iterator, 0, len(groups)-1)
-		// NOTE: If this panics, there is a mismatch with groups. Uncomment the following line to debug.
+		// NOTE: If this panics, there is a mismatch with groups
 		for i, group := range groups[1:] {
 			iterators = append(iterators, emitters[i].Emit([]string{group}, lexer))
 		}
@@ -41,6 +41,74 @@ func ByGroups(emitters ...Emitter) Emitter {
 	})
 }
 
+// UsingByGroup emits tokens for the matched groups in the regex using a
+// "sublexer". Used when lexing code blocks where the name of a sublexer is
+// contained within the block, for example on a Markdown text block or SQL
+// language block.
+//
+// The sublexer will be retrieved using sublexerGetFunc (typically
+// internal.Get), using the captured value from the matched sublexerNameGroup.
+//
+// If sublexerGetFunc returns a non-nil lexer for the captured sublexerNameGroup,
+// then tokens for the matched codeGroup will be emitted using the retrieved
+// lexer. Otherwise, if the sublexer is nil, then tokens will be emitted from
+// the passed emitter.
+//
+// Example:
+//
+//	var Markdown = internal.Register(MustNewLexer(
+//		&Config{
+//			Name:      "markdown",
+//			Aliases:   []string{"md", "mkd"},
+//			Filenames: []string{"*.md", "*.mkd", "*.markdown"},
+//			MimeTypes: []string{"text/x-markdown"},
+//		},
+//		Rules{
+//			"root": {
+//				{"^(```)(\\w+)(\\n)([\\w\\W]*?)(^```$)",
+//					UsingByGroup(
+//						internal.Get,
+//						2, 4,
+//						String, String, String, Text, String,
+//					),
+//					nil,
+//				},
+//			},
+//		},
+//	))
+//
+// See the lexers/m/markdown.go for the complete example.
+//
+// Note: panic's if the number emitters does not equal the number of matched
+// groups in the regex.
+func UsingByGroup(sublexerGetFunc func(string) Lexer, sublexerNameGroup, codeGroup int, emitters ...Emitter) Emitter {
+	return EmitterFunc(func(groups []string, lexer Lexer) Iterator {
+		// bounds check
+		if len(emitters) != len(groups)-1 {
+			panic("UsingByGroup expects number of emitters to be the same as len(groups)-1")
+		}
+
+		// grab sublexer
+		sublexer := sublexerGetFunc(groups[sublexerNameGroup])
+
+		// build iterators
+		iterators := make([]Iterator, len(groups)-1)
+		for i, group := range groups[1:] {
+			if i == codeGroup-1 && sublexer != nil {
+				var err error
+				iterators[i], err = sublexer.Tokenise(nil, groups[codeGroup])
+				if err != nil {
+					panic(err)
+				}
+			} else {
+				iterators[i] = emitters[i].Emit([]string{group}, lexer)
+			}
+		}
+
+		return Concaterator(iterators...)
+	})
+}
+
 // Using returns an Emitter that uses a given Lexer for parsing and emitting.
 func Using(lexer Lexer) Emitter {
 	return EmitterFunc(func(groups []string, _ Lexer) Iterator {