4 files changed, 16 insertions, 12 deletions
diff --git a/lib/impure/nre.nim b/lib/impure/nre.nim
index 58594f054..4ceab2cca 100644
--- a/lib/impure/nre.nim
+++ b/lib/impure/nre.nim
@@ -289,7 +289,7 @@ proc `[]`*(pattern: Captures, i: int): string =
     let bounds = bounds.get
     return pattern.str.substr(bounds.a, bounds.b)
   else:
-    return ""
+    return nil
 
 proc match*(pattern: RegexMatch): string =
   return pattern.captures[-1]
@@ -313,9 +313,9 @@ template toTableImpl(cond: untyped) {.dirty.} =
     else:
       result[key] = nextVal
 
-proc toTable*(pattern: Captures, default: string = ""): Table[string, string] =
+proc toTable*(pattern: Captures, default: string = nil): Table[string, string] =
   result = initTable[string, string]()
-  toTableImpl(nextVal.len == 0)
+  toTableImpl(nextVal == nil)
 
 proc toTable*(pattern: CaptureBounds, default = none(HSlice[int, int])):
     Table[string, Option[HSlice[int, int]]] =
@@ -334,13 +334,13 @@ template itemsImpl(cond: untyped) {.dirty.} =
 iterator items*(pattern: CaptureBounds, default = none(HSlice[int, int])): Option[HSlice[int, int]] =
   itemsImpl(nextVal.isNone)
 
-iterator items*(pattern: Captures, default: string = ""): string =
-  itemsImpl(nextVal.len == 0)
+iterator items*(pattern: Captures, default: string = nil): string =
+  itemsImpl(nextVal == nil)
 
 proc toSeq*(pattern: CaptureBounds, default = none(HSlice[int, int])): seq[Option[HSlice[int, int]]] =
   accumulateResult(pattern.items(default))
 
-proc toSeq*(pattern: Captures, default: string = ""): seq[string] =
+proc toSeq*(pattern: Captures, default: string = nil): seq[string] =
   accumulateResult(pattern.items(default))
 
 proc `$`*(pattern: RegexMatch): string =
diff --git a/lib/impure/nre/private/util.nim b/lib/impure/nre/private/util.nim
index a3ae84007..12d2506ea 100644
--- a/lib/impure/nre/private/util.nim
+++ b/lib/impure/nre/private/util.nim
@@ -10,7 +10,11 @@ proc fget*[K, V](self: Table[K, V], key: K): V =
 const Ident = {'a'..'z', 'A'..'Z', '0'..'9', '_', '\128'..'\255'}
 const StartIdent = Ident - {'0'..'9'}
 
-template checkNil(arg: string): string = arg
+proc checkNil(arg: string): string =
+  if arg == nil:
+    raise newException(ValueError, "Cannot use nil capture")
+  else:
+    return arg
 
 template formatStr*(howExpr, namegetter, idgetter): untyped =
   let how = howExpr
diff --git a/tests/stdlib/nre/captures.nim b/tests/stdlib/nre/captures.nim
index 31de71154..19c344a8d 100644
--- a/tests/stdlib/nre/captures.nim
+++ b/tests/stdlib/nre/captures.nim
@@ -27,7 +27,7 @@ suite "captures":
 
     let ex2 = "foo".find(re("(?<foo>foo)(?<bar>bar)?"))
     check(ex2.captures["foo"] == "foo")
-    check(ex2.captures["bar"] == "")
+    check(ex2.captures["bar"] == nil)
 
   test "named capture bounds":
     let ex1 = "foo".find(re("(?<foo>foo)(?<bar>bar)?"))
@@ -41,7 +41,7 @@ suite "captures":
 
   test "named capture table":
     let ex1 = "foo".find(re("(?<foo>foo)(?<bar>bar)?"))
-    check(ex1.captures.toTable == {"foo" : "foo", "bar" : ""}.toTable())
+    check(ex1.captures.toTable == {"foo" : "foo", "bar" : nil}.toTable())
     check(ex1.captureBounds.toTable == {"foo" : some(0..2), "bar" : none(Slice[int])}.toTable())
     check(ex1.captures.toTable("") == {"foo" : "foo", "bar" : ""}.toTable())
 
@@ -50,7 +50,7 @@ suite "captures":
 
   test "capture sequence":
     let ex1 = "foo".find(re("(?<foo>foo)(?<bar>bar)?"))
-    check(ex1.captures.toSeq == @["foo", ""])
+    check(ex1.captures.toSeq == @["foo", nil])
     check(ex1.captureBounds.toSeq == @[some(0..2), none(Slice[int])])
     check(ex1.captures.toSeq("") == @["foo", ""])
 
diff --git a/tests/stdlib/nre/replace.nim b/tests/stdlib/nre/replace.nim
index b762271a2..516fd4328 100644
--- a/tests/stdlib/nre/replace.nim
+++ b/tests/stdlib/nre/replace.nim
@@ -16,5 +16,5 @@ suite "replace":
     check("123".replace(re"(?<foo>\d)(\d)", "${foo}$#$#") == "1123")
 
   test "replacing missing captures should throw instead of segfaulting":
-    discard "ab".replace(re"(a)|(b)", "$1$2")
-    discard "b".replace(re"(a)?(b)", "$1$2")
+    expect ValueError: discard "ab".replace(re"(a)|(b)", "$1$2")
+    expect ValueError: discard "b".replace(re"(a)?(b)", "$1$2")