diff --git a/src/core/operations/ToCaseInsensitiveRegex.mjs b/src/core/operations/ToCaseInsensitiveRegex.mjs
index 28bd3dc9..77b14172 100644
--- a/src/core/operations/ToCaseInsensitiveRegex.mjs
+++ b/src/core/operations/ToCaseInsensitiveRegex.mjs
@@ -1,10 +1,12 @@
 /**
  * @author masq [github.cyberchef@masq.cc]
+ * @author n1073645
  * @copyright Crown Copyright 2018
  * @license Apache-2.0
  */
 
 import Operation from "../Operation.mjs";
+import OperationError from "../errors/OperationError.mjs";
 
 /**
  * To Case Insensitive Regex operation
@@ -32,7 +34,61 @@ class ToCaseInsensitiveRegex extends Operation {
      * @returns {string}
      */
     run(input, args) {
-        return input.replace(/[a-z]/ig, m => `[${m.toLowerCase()}${m.toUpperCase()}]`);
+
+        /**
+         * Simulates look behind behaviour since javascript doesn't support it.
+         *
+         * @param {string} input
+         * @returns {string}
+         */
+        function preProcess(input) {
+            let result = "";
+            for (let i = 0; i < input.length; i++) {
+                const temp = input.charAt(i);
+                if (temp.match(/[a-zA-Z]/g) && (input.charAt(i-1) !== "-") && (input.charAt(i+1) !== "-"))
+                    result += "[" + temp.toLowerCase() + temp.toUpperCase() + "]";
+                else
+                    result += temp;
+            }
+            return result;
+        }
+
+        try {
+            RegExp(input);
+        } catch (error) {
+            throw new OperationError("Invalid Regular Expression (Please note this version of node does not support look behinds).");
+        }
+
+        // Example: [test] -> [[tT][eE][sS][tT]]
+        return preProcess(input)
+
+            // Example: [A-Z] -> [A-Za-z]
+            .replace(/([A-Z]-[A-Z]|[a-z]-[a-z])/g, m => `${m[0].toUpperCase()}-${m[2].toUpperCase()}${m[0].toLowerCase()}-${m[2].toLowerCase()}`)
+
+            // Example: [H-d] -> [A-DH-dh-z]
+            .replace(/[A-Z]-[a-z]/g, m => `A-${m[2].toUpperCase()}${m}${m[0].toLowerCase()}-z`)
+
+            // Example: [!-D] -> [!-Da-d]
+            .replace(/\\?[ -@]-[A-Z]/g, m => `${m}a-${m[2].toLowerCase()}`)
+
+            // Example: [%-^] -> [%-^a-z]
+            .replace(/\\?[ -@]-\\?[[-`]/g, m => `${m}a-z`)
+
+            // Example: [K-`] -> [K-`k-z]
+            .replace(/[A-Z]-\\?[[-`]/g, m => `${m}${m[0].toLowerCase()}-z`)
+
+            // Example: [[-}] -> [[-}A-Z]
+            .replace(/\\?[[-`]-\\?[{-~]/g, m => `${m}A-Z`)
+
+            // Example: [b-}] -> [b-}B-Z]
+            .replace(/[a-z]-\\?[{-~]/g, m => `${m}${m[0].toUpperCase()}-Z`)
+
+            // Example: [<-j] -> [<-z]
+            .replace(/\\?[ -@]-[a-z]/g, m => `${m[0]}-z`)
+
+            // Example: [^-j] -> [A-J^-j]
+            .replace(/\\?[[-`]-[a-z]/g, m => `A-${m[2].toUpperCase()}${m}`);
+
     }
 }
 
diff --git a/tests/operations/tests/ToFromInsensitiveRegex.mjs b/tests/operations/tests/ToFromInsensitiveRegex.mjs
index b74e9973..1394bfe8 100644
--- a/tests/operations/tests/ToFromInsensitiveRegex.mjs
+++ b/tests/operations/tests/ToFromInsensitiveRegex.mjs
@@ -53,4 +53,136 @@ TestRegister.addTests([
             },
         ],
     },
+    {
+        name: "To Case Insensitive Regex: [A-Z] -> [A-Za-z]",
+        input: "[A-Z]",
+        expectedOutput: "[A-Za-z]",
+        recipeConfig: [
+            {
+                op: "To Case Insensitive Regex",
+                args: [],
+            },
+        ],
+    },
+    {
+        name: "To Case Insensitive Regex: [a-z] -> [A-Za-z]",
+        input: "[a-z]",
+        expectedOutput: "[A-Za-z]",
+        recipeConfig: [
+            {
+                op: "To Case Insensitive Regex",
+                args: [],
+            },
+        ],
+    },
+    {
+        name: "To Case Insensitive Regex: [H-d] -> [A-DH-dh-z]",
+        input: "[H-d]",
+        expectedOutput: "[A-DH-dh-z]",
+        recipeConfig: [
+            {
+                op: "To Case Insensitive Regex",
+                args: [],
+            },
+        ],
+    },
+    {
+        name: "To Case Insensitive Regex: [!-D] -> [!-Da-d]",
+        input: "[!-D]",
+        expectedOutput: "[!-Da-d]",
+        recipeConfig: [
+            {
+                op: "To Case Insensitive Regex",
+                args: [],
+            },
+        ],
+    },
+    {
+        name: "To Case Insensitive Regex: [%-^] -> [%-^a-z]",
+        input: "[%-^]",
+        expectedOutput: "[%-^a-z]",
+        recipeConfig: [
+            {
+                op: "To Case Insensitive Regex",
+                args: [],
+            },
+        ],
+    },
+    {
+        name: "To Case Insensitive Regex: [K-`] -> [K-`k-z]",
+        input: "[K-`]",
+        expectedOutput: "[K-`k-z]",
+        recipeConfig: [
+            {
+                op: "To Case Insensitive Regex",
+                args: [],
+            },
+        ],
+    },
+    {
+        name: "To Case Insensitive Regex: [[-}] -> [[-}A-Z]",
+        input: "[[-}]",
+        expectedOutput: "[[-}A-Z]",
+        recipeConfig: [
+            {
+                op: "To Case Insensitive Regex",
+                args: [],
+            },
+        ],
+    },
+    {
+        name: "To Case Insensitive Regex: [b-}] -> [b-}B-Z]",
+        input: "[b-}]",
+        expectedOutput: "[b-}B-Z]",
+        recipeConfig: [
+            {
+                op: "To Case Insensitive Regex",
+                args: [],
+            },
+        ],
+    },
+    {
+        name: "To Case Insensitive Regex: [<-j] -> [<-z]",
+        input: "[<-j]",
+        expectedOutput: "[<-z]",
+        recipeConfig: [
+            {
+                op: "To Case Insensitive Regex",
+                args: [],
+            },
+        ],
+    },
+    {
+        name: "To Case Insensitive Regex: [^-j] -> [A-J^-j]",
+        input: "[^-j]",
+        expectedOutput: "[A-J^-j]",
+        recipeConfig: [
+            {
+                op: "To Case Insensitive Regex",
+                args: [],
+            },
+        ],
+    },
+    {
+        name: "To Case Insensitive Regex: not simple test",
+        input: "Mozilla[A-Z0-9]+[A-Z]Mozilla[0-9whatA-Z][H-d][!-H][a-~](.)+",
+        expectedOutput: "[mM][oO][zZ][iI][lL][lL][aA][A-Za-z0-9]+[A-Za-z][mM][oO][zZ][iI][lL][lL][aA][0-9[wW][hH][aA][tT]A-Za-z][A-DH-dh-z][!-Ha-h][a-~A-Z](.)+",
+        recipeConfig: [
+            {
+                op: "To Case Insensitive Regex",
+                args: [],
+            },
+        ],
+    },
+    {
+        name: "To Case Insensitive Regex: erroneous test",
+        input: "Mozilla[A-Z",
+        expectedOutput: "Invalid Regular Expression (Please note this version of node does not support look behinds).",
+        recipeConfig: [
+            {
+                op: "To Case Insensitive Regex",
+                args: [],
+            },
+        ],
+    }
 ]);