diff --git a/lib/functions.js b/lib/functions.js
index e957c8c..cbeaea4 100644
--- a/lib/functions.js
+++ b/lib/functions.js
@@ -401,6 +401,94 @@ registerFunction({
   }
 })
 
+registerFunction({
+  name: 'indexOf',
+  instances: ['String'],
+  returns: ['Integer'],
+  minArgs: 2,
+  maxArgs: 3,
+  handler(string, search, fromIndex) {
+    if (typeof search !== 'string') {
+      throw new EvaluationError('string.indexOf(search, fromIndex): search must be a string')
+    }
+
+    if (fromIndex === undefined) return BigInt(string.indexOf(search))
+
+    if (typeof fromIndex !== 'bigint') {
+      throw new EvaluationError('string.indexOf(search, fromIndex): fromIndex must be an integer')
+    }
+
+    if (search === '') return fromIndex
+
+    fromIndex = Number(fromIndex)
+    if (fromIndex < 0 || fromIndex >= string.length) {
+      throw new EvaluationError('string.indexOf(search, fromIndex): fromIndex out of range')
+    }
+
+    return BigInt(string.indexOf(search, fromIndex))
+  }
+})
+
+registerFunction({
+  name: 'lastIndexOf',
+  instances: ['String'],
+  returns: ['Integer'],
+  minArgs: 2,
+  maxArgs: 3,
+  handler(string, search, fromIndex) {
+    if (typeof search !== 'string') {
+      throw new EvaluationError('string.lastIndexOf(search, fromIndex): search must be a string')
+    }
+
+    if (fromIndex === undefined) return BigInt(string.lastIndexOf(search))
+
+    if (typeof fromIndex !== 'bigint') {
+      throw new EvaluationError(
+        'string.lastIndexOf(search, fromIndex): fromIndex must be an integer'
+      )
+    }
+
+    if (search === '') return fromIndex
+
+    fromIndex = Number(fromIndex)
+    if (fromIndex < 0 || fromIndex >= string.length) {
+      throw new EvaluationError('string.lastIndexOf(search, fromIndex): fromIndex out of range')
+    }
+
+    return BigInt(string.lastIndexOf(search, fromIndex))
+  }
+})
+
+registerFunction({
+  name: 'substring',
+  instances: ['String'],
+  returns: ['String'],
+  minArgs: 2,
+  maxArgs: 3,
+  handler(string, start, end) {
+    if (typeof start === 'bigint') start = Number(start)
+
+    if (typeof start !== 'number') {
+      throw new EvaluationError('string.substring(start, end): start index must be an integer')
+    }
+
+    if (start < 0 || start > string.length) {
+      throw new EvaluationError('string.substring(start, end): start index out of range')
+    }
+
+    if (end === undefined) return string.substring(start)
+
+    if (typeof end === 'bigint') end = Number(end)
+    if (typeof end !== 'number') {
+      throw new EvaluationError('string.substring(start, end): end index must be an integer')
+    } else if (end < start || end > string.length) {
+      throw new EvaluationError('string.substring(start, end): end index out of range')
+    }
+
+    return string.substring(start, end)
+  }
+})
+
 registerFunction({
   name: 'matches',
   instances: ['String'],
@@ -419,6 +507,46 @@ registerFunction({
   }
 })
 
+registerFunction({
+  name: 'split',
+  instances: ['String'],
+  returns: ['List'],
+  minArgs: 2,
+  maxArgs: 3,
+  handler(string, separator, limit) {
+    if (limit !== undefined && !(typeof limit === 'bigint' || typeof limit === 'number')) {
+      throw new EvaluationError('string.split(separator, limit): limit must be an integer')
+    }
+
+    if (typeof separator !== 'string') {
+      throw new EvaluationError('string.split(separator, limit): separator must be a string')
+    }
+
+    return string.split(separator, limit)
+  }
+})
+
+registerFunction({
+  name: 'join',
+  instances: ['List'],
+  returns: ['String'],
+  minArgs: 1,
+  maxArgs: 2,
+  handler(list, separator = '') {
+    if (typeof separator !== 'string') {
+      throw new EvaluationError('string.join(separator): separator must be a string')
+    }
+
+    for (let i = 0; i < list.length; i++) {
+      if (typeof list[i] !== 'string') {
+        throw new EvaluationError('string.join(separator): list must contain only strings')
+      }
+    }
+
+    return list.join(separator)
+  }
+})
+
 const ByteOpts =
   typeof Buffer !== 'undefined'
     ? {