diff --git a/effekt/js/src/main/scala/effekt/context/VirtualModuleDB.scala b/effekt/js/src/main/scala/effekt/context/VirtualModuleDB.scala
index c154f20c1..4dbf27700 100644
--- a/effekt/js/src/main/scala/effekt/context/VirtualModuleDB.scala
+++ b/effekt/js/src/main/scala/effekt/context/VirtualModuleDB.scala
@@ -17,11 +17,9 @@ trait VirtualModuleDB extends ModuleDB { self: Context =>
    * used by Namer to resolve FFI includes
    */
   override def contentsOf(path: String): Option[String] = {
-    val f = file(module.source.name).parent / path
-    if (!f.exists) {
-      None
-    } else {
-      Some(f.read)
+    val parent = file(module.source.name).parent
+    (parent :: config.includes().map(file)).collectFirst {
+      case base if (base / path).exists => (base / path).read
     }
   }
 
diff --git a/effekt/jvm/src/main/scala/effekt/context/IOModuleDB.scala b/effekt/jvm/src/main/scala/effekt/context/IOModuleDB.scala
index bc8781998..4d3ac96ea 100644
--- a/effekt/jvm/src/main/scala/effekt/context/IOModuleDB.scala
+++ b/effekt/jvm/src/main/scala/effekt/context/IOModuleDB.scala
@@ -13,11 +13,9 @@ trait IOModuleDB extends ModuleDB { self: Context =>
    * used by Namer to resolve FFI includes
    */
   override def contentsOf(path: String): Option[String] = {
-    val includeFile = file(module.source.name).parent / path
-    if (!includeFile.exists) {
-      None
-    } else {
-      Some(FileSource(includeFile.toString).content)
+    val parent = file(module.source.name).parent
+    (parent :: config.includes().map(file)).collectFirst {
+      case base if (base / path).exists => FileSource((base / path).toString).content
     }
   }
 
diff --git a/effekt/shared/src/main/scala/effekt/core/vm/Builtin.scala b/effekt/shared/src/main/scala/effekt/core/vm/Builtin.scala
index 95a7bf26a..1b3209fa3 100644
--- a/effekt/shared/src/main/scala/effekt/core/vm/Builtin.scala
+++ b/effekt/shared/src/main/scala/effekt/core/vm/Builtin.scala
@@ -262,6 +262,12 @@ lazy val arrays: Builtins = Map(
   },
 )
 
+lazy val undefined: Builtins = Map(
+  builtin("effekt::isUndefined[A](A)") {
+    case Value.Literal(m) :: Nil => Value.Bool(m == null)
+  },
+)
+
 lazy val refs: Builtins = Map(
   builtin("ref::ref[T](T)") {
     case init :: Nil => Value.Ref(Reference(init))
@@ -281,9 +287,6 @@ lazy val regexes: Builtins = Map(
   builtin("regex::exec(Regex, String)") {
     case As.Regex(r) :: As.String(str) :: Nil => Value.Literal(r.findFirstMatchIn(str).orNull)
   },
-  builtin("regex::isDefined(RegexMatch)") {
-    case As.RegexMatch(m) :: Nil => Value.Bool(m != null)
-  },
   builtin("regex::matched(RegexMatch)") {
     case As.RegexMatch(m) :: Nil => Value.String(m.matched)
   },
@@ -292,7 +295,7 @@ lazy val regexes: Builtins = Map(
   },
 )
 
-lazy val builtins: Builtins = printing ++ integers ++ doubles ++ booleans ++ strings ++ arrays ++ refs ++ chars ++ regexes
+lazy val builtins: Builtins = printing ++ integers ++ doubles ++ booleans ++ strings ++ arrays ++ refs ++ chars ++ regexes ++ undefined
 
 protected object As {
   object String {
diff --git a/examples/chez/libraries.effekt b/examples/chez/libraries.effekt
index 8b1d2b532..15b26cd91 100644
--- a/examples/chez/libraries.effekt
+++ b/examples/chez/libraries.effekt
@@ -12,7 +12,7 @@ import array
 import mutable/dict
 import ref
 
-import text/regex
+import regex
 import string
 
 def main() = ()
diff --git a/examples/neg/unbound_type.effekt b/examples/neg/unbound_type.effekt
index 160767fb5..48c63cb50 100644
--- a/examples/neg/unbound_type.effekt
+++ b/examples/neg/unbound_type.effekt
@@ -1,7 +1,7 @@
 module examples/pos/lexer
 
 import string
-import text/regex
+import regex
 
 effect EOS(): Nothing
 effect LexerError(msg: String, pos: Position): Nothing
diff --git a/examples/pos/simpleparser.effekt b/examples/pos/simpleparser.effekt
index 09d7ef5c7..1cd69464d 100644
--- a/examples/pos/simpleparser.effekt
+++ b/examples/pos/simpleparser.effekt
@@ -1,7 +1,7 @@
 module examples/pos/simpleparser
 
 import string
-import text/regex
+import regex
 
 effect fail(msg: String): Nothing
 
diff --git a/libraries/chez/common/text/regex.effekt b/libraries/chez/common/text/regex.effekt
deleted file mode 100644
index f9bea3ce4..000000000
--- a/libraries/chez/common/text/regex.effekt
+++ /dev/null
@@ -1,31 +0,0 @@
-module text/regex
-
-import immutable/cslist
-
-extern include chez "pregexp.scm"
-
-extern type Regex
-
-record Match(matched: String, index: Int)
-
-extern pure def regex(str: String): Regex =
-  chez "(pregexp ${str})"
-
-def exec(reg: Regex, str: String): Option[Match] = {
-  val matched = reg.unsafeMatchString(str)
-  if (matched.isUndefined) { None() }
-  else { Some(Match(matched, reg.unsafeMatchIndex(str))) }
-}
-
-extern pure def unsafeMatchIndex(reg: Regex, str: String): Int =
-  chez "(let ([m (pregexp-match-positions ${reg} ${str})]) (if m (car (car m)) #f))"
-
-// we ignore the captures for now and only return the whole match
-extern pure def unsafeMatchString(reg: Regex, str: String): String =
-  chez "(let ([m (pregexp-match ${reg} ${str})]) (if m (car m) #f))"
-
-extern pure def split(reg: Regex, str: String): CSList[String] =
-  chez "(pregexp-split ${reg} ${str})"
-
-// def split(str: String, sep: String): Array[String] =
-//   toArray(split(sep.regex, str))
\ No newline at end of file
diff --git a/libraries/common/effekt.effekt b/libraries/common/effekt.effekt
index 2049fb3b8..96fbfac18 100644
--- a/libraries/common/effekt.effekt
+++ b/libraries/common/effekt.effekt
@@ -667,11 +667,13 @@ extern pure def toInt(n: Byte): Int =
 extern pure def undefined[A](): A =
   js "undefined"
   chez "#f"
+  vm "effekt::undefined()"
 
 /// Is an FFI value undefined?
 extern pure def isUndefined[A](value: A): Bool =
   js "(${value} === undefined || ${value} === null)"
   chez "(eq? ${value} #f)"
+  vm "effekt::isUndefined[A](A)"
 
 
 // Tuples
diff --git a/libraries/common/regex.effekt b/libraries/common/regex.effekt
index ca3901bda..077f9409d 100644
--- a/libraries/common/regex.effekt
+++ b/libraries/common/regex.effekt
@@ -2,44 +2,59 @@ module regex
 
 import string
 
+extern include chez "text/pregexp.scm"
+
 extern type Regex
 
 record Match(matched: String, index: Int)
 
 extern pure def regex(str: String): Regex =
   js "new RegExp(${str})"
+  chez "(pregexp ${str})"
   vm "regex::regex(String)"
 
 def exec(reg: Regex, str: String): Option[Match] = {
   val v = reg.unsafeExec(str)
-  if (v.isDefined)
-    Some(Match(v.matched, v.index))
-  else
+  if (v.isUndefined)
     None()
+  else
+    Some(Match(v.matched, v.index))
 }
 
 extern type RegexMatch
   // js: { matched: String, index: Int } | undefined
   // vm: scala.util.matching.Regex.Match | null
-extern pure def isDefined(r: RegexMatch): Bool =
-  js "!!${r}"
-  vm "regex::isDefined(RegexMatch)"
+
 extern pure def matched(r: RegexMatch): String =
   js "${r}.matched"
+  chez "(vector-ref ${r} 0)"
   vm "regex::matched(RegexMatch)"
+
 extern pure def index(r: RegexMatch): Int =
   js "${r}.index"
+  chez "(vector-ref ${r} 1)"
   vm "regex::index(RegexMatch)"
 
 extern js """
-function regex$exec(reg, str) {
-  var res = reg.exec(str);
-  if (res === null) { return undefined }
-  else { return { matched: res[0], index: res.index } }
-}
+  function regex$exec(reg, str) {
+    var res = reg.exec(str);
+    if (res === null) { return undefined }
+    else { return { matched: res[0], index: res.index } }
+  }
+"""
+
+extern chez """
+(define regex-exec
+  (lambda (regex str)
+    (let* ([positions (pregexp-match-positions regex str)]
+       [match (pregexp-match regex str)])
+  (if (and positions match)
+      (vector (car match) (caar positions))
+      #f))))
 """
 
 // internals
 extern io def unsafeExec(reg: Regex, str: String): RegexMatch =
   js "regex$exec(${reg}, ${str})"
+  chez "(regex-exec ${reg} ${str})"
   vm "regex::exec(Regex, String)"