diff --git a/README.md b/README.md
index fd2a3e4..4b78d01 100644
--- a/README.md
+++ b/README.md
@@ -79,6 +79,18 @@ console.log(helloStr)
 
 Check out the [math lib](./src/lib/math.ts) for a more extensive example.
 
+### Extend a library
+
+Use `extendLib` to add new functions to an existing library without overwriting it.
+
+```js
+const extraMath = new luainjs.Table({ tau: Math.PI * 2 })
+luaEnv.extendLib('math', extraMath)
+
+const tau = luaEnv.parse('return math.tau').exec()
+console.log(tau)
+```
+
 ## Example
 
 Check out the [test runner](./tests/test.js) for a concrete example.
diff --git a/src/index.ts b/src/index.ts
index 4d7b72f..5556d0e 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -11,7 +11,7 @@ import { libString, metatable as stringMetatable } from './lib/string'
 import { getLibOS } from './lib/os'
 import { getLibPackage } from './lib/package'
 import { libCoroutine } from './lib/coroutine'
-import { LuaType, ensureArray, Config } from './utils'
+import { LuaType, ensureArray, Config, hasOwnProperty } from './utils'
 import { Thread } from './Thread'
 import { parse as parseScript } from './parser'
 
@@ -72,6 +72,7 @@ function createEnv(
     parse: (script: string) => Script
     parseFile: (path: string) => Script
     loadLib: (name: string, value: Table) => void
+    extendLib: (name: string, value: Table) => void
 } {
     const cfg: Config = {
         LUA_PATH: './?.lua',
@@ -93,6 +94,24 @@ function createEnv(
         loaded.rawset(name, value)
     }
 
+    const extendLib = (name: string, value: Table): void => {
+        const lib = _G.rawget(name)
+        if (lib instanceof Table) {
+            for (let i = 1; i < value.numValues.length; i++) {
+                if (hasOwnProperty(value.numValues, i)) lib.rawset(i, value.numValues[i])
+            }
+            for (const key in value.strValues) {
+                if (hasOwnProperty(value.strValues, key)) lib.rawset(key, value.strValues[key])
+            }
+            for (let i = 0; i < value.keys.length; i++) {
+                lib.rawset(value.keys[i], value.values[i])
+            }
+            return
+        }
+
+        loadLib(name, value)
+    }
+
     loadLib('_G', _G)
     loadLib('package', libPackage)
     loadLib('math', libMath)
@@ -122,7 +141,8 @@ function createEnv(
     return {
         parse,
         parseFile,
-        loadLib
+        loadLib,
+        extendLib
     }
 }
 
diff --git a/src/lib/string.ts b/src/lib/string.ts
index d504ac1..3107dc6 100644
--- a/src/lib/string.ts
+++ b/src/lib/string.ts
@@ -1,7 +1,15 @@
 import { sprintf } from 'printj'
 import { Table } from '../Table'
 import { LuaError } from '../LuaError'
-import { tostring, posrelat, coerceArgToNumber, coerceArgToString, hasOwnProperty, LuaType } from '../utils'
+import {
+    coerceArgToBoolean,
+    coerceArgToNumber,
+    coerceArgToString,
+    hasOwnProperty,
+    LuaType,
+    posrelat,
+    tostring
+} from '../utils'
 
 const ROSETTA_STONE: Record<string, string> = {
     '([^a-zA-Z0-9%(])-': '$1*?',
@@ -117,7 +125,7 @@ function find(s: LuaType, pattern: LuaType, init: LuaType, plain: LuaType): (num
     const S = coerceArgToString(s, 'find', 1)
     const P = coerceArgToString(pattern, 'find', 2)
     const INIT = init === undefined ? 1 : coerceArgToNumber(init, 'find', 3)
-    const PLAIN = plain === undefined ? false : coerceArgToNumber(plain, 'find', 4)
+    const PLAIN = plain === undefined ? false : coerceArgToBoolean(plain, 'find', 4)
 
     // Regex
     if (!PLAIN) {
diff --git a/src/utils.ts b/src/utils.ts
index ee2a8e6..21ab97a 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -166,7 +166,7 @@ function coerceToString(val: LuaType, errorMessage?: string): string {
 function coerceArg<T>(
     value: LuaType,
     coerceFunc: (val: LuaType, errorMessage?: string) => T,
-    typ: 'number' | 'string',
+    typ: 'number' | 'string' | 'boolean',
     funcName: string,
     index: number
 ): T {
@@ -181,6 +181,10 @@ function coerceArgToString(value: LuaType, funcName: string, index: number): str
     return coerceArg<string>(value, coerceToString, 'string', funcName, index)
 }
 
+function coerceArgToBoolean(value: LuaType, funcName: string, index: number): boolean {
+    return coerceArg<boolean>(value, coerceToBoolean, 'boolean', funcName, index)
+}
+
 function coerceArgToTable(value: LuaType, funcName: string, index: number): Table {
     if (value instanceof Table) {
         return value
@@ -221,10 +225,11 @@ export {
     coerceToBoolean,
     coerceToNumber,
     coerceToString,
+    coerceArgToBoolean,
+    coerceArgToFunction,
     coerceArgToNumber,
     coerceArgToString,
     coerceArgToTable,
-    coerceArgToFunction,
     coerceArgToThread,
     ensureArray,
     hasOwnProperty
diff --git a/tests/starlight/lib-string.lua b/tests/starlight/lib-string.lua
index 59f9749..a89e55f 100644
--- a/tests/starlight/lib-string.lua
+++ b/tests/starlight/lib-string.lua
@@ -101,6 +101,13 @@ b = string.find(a, 'The .* fox')
 assertTrue (b == 1, 'The dot pattern should match across lines in string.find()')
 
 
+local ok, plainFind = pcall(function () return string.find('abc', 'a.c', 1, true) end)
+assertTrue (ok and plainFind == nil, 'string.find() should accept a boolean plain argument and disable pattern matching')
+
+local literalFind = string.find('a.c', 'a.c', 1, true)
+assertTrue (literalFind == 1, 'string.find() should find literal matches when plain is true')
+
+
 
 -- format
 
diff --git a/tests/test.js b/tests/test.js
index bc27821..f56f424 100644
--- a/tests/test.js
+++ b/tests/test.js
@@ -41,6 +41,16 @@ let exitCode = 0
     }
 }
 
+{
+    const luaEnv = luainjs.createEnv()
+    const ext = new luainjs.Table({ foo: () => 'bar' })
+    luaEnv.extendLib('math', ext)
+    const val = luaEnv.parse('return math.foo()').exec()
+    if (val !== 'bar') {
+        throw Error('extendLib failed!')
+    }
+}
+
 {
     const luaEnv = luainjs.createEnv()
     let str