diff --git a/packages/format-library/src/code/index.js b/packages/format-library/src/code/index.js
index f3651504d57ce8..27a99ebc989e21 100644
--- a/packages/format-library/src/code/index.js
+++ b/packages/format-library/src/code/index.js
@@ -20,16 +20,18 @@ export const code = {
 	__unstableInputRule( value ) {
 		const BACKTICK = '`';
 		const { start, text } = value;
-		const characterBefore = text.slice( start - 1, start );
+		const characterBefore = text[ start - 1 ];
 
 		// Quick check the text for the necessary character.
 		if ( characterBefore !== BACKTICK ) {
 			return value;
 		}
 
-		const textBefore = text.slice( 0, start - 1 );
-		const indexBefore = textBefore.lastIndexOf( BACKTICK );
+		if ( start - 2 < 0 ) {
+			return value;
+		}
 
+		const indexBefore = text.lastIndexOf( BACKTICK, start - 2 );
 		if ( indexBefore === -1 ) {
 			return value;
 		}