Throw an exception when replacing with a nil value

README.asciidoc

@@ -95,6 +95,8 @@ If `sub` is a string, the syntax is as follows:
 - `$#` - first capture
 - `$0` - full match
 
+If a given capture is missing, a `ValueError` exception is thrown.
+
 [[proc-escapere]]
 ==== escapeRe(string): string
 

src/private/util.nim

@@ -9,6 +9,12 @@ 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'}
 
+proc checkNil(arg: string): string =
+  if arg == nil:
+    raise newException(ValueError, "Cannot use nil capture")
+  else:
+    return arg
+
 template formatStr*(howExpr, namegetter, idgetter: expr): expr =
   let how = howExpr
   var val = newStringOfCap(how.len)
@@ -19,37 +25,38 @@ template formatStr*(howExpr, namegetter, idgetter: expr): expr =
     if how[i] != '$':
       val.add(how[i])
       i += 1
-    elif how[i + 1] == '$':
-      val.add('$')
-      i += 2
-    elif how[i + 1] == '#':
-      var id {.inject.} = lastNum
-      val.add(idgetter)
-      lastNum += 1
-      i += 2
-    elif how[i + 1] in {'0'..'9'}:
-      i += 1
-      var id {.inject.} = 0
-      while i < how.len and how[i] in {'0'..'9'}:
-        id += (id * 10) + (ord(how[i]) - ord('0'))
-        i += 1
-      val.add(idgetter)
-      lastNum = id + 1
-    elif how[i + 1] in StartIdent:
-      i += 1
-      var name {.inject.} = ""
-      while i < how.len and how[i] in Ident:
-        name.add(how[i])
-        i += 1
-      val.add(namegetter)
-    elif how[i + 1] == '{':
-      i += 2
-      var name {.inject.} = ""
-      while i < how.len and how[i] != '}':
-        name.add(how[i])
-        i += 1
-      i += 1
-      val.add(namegetter)
     else:
-      raise newException(Exception, "Syntax error in format string at " & $i)
+      if how[i + 1] == '$':
+        val.add('$')
+        i += 2
+      elif how[i + 1] == '#':
+        var id {.inject.} = lastNum
+        val.add(checkNil(idgetter))
+        lastNum += 1
+        i += 2
+      elif how[i + 1] in {'0'..'9'}:
+        i += 1
+        var id {.inject.} = 0
+        while i < how.len and how[i] in {'0'..'9'}:
+          id += (id * 10) + (ord(how[i]) - ord('0'))
+          i += 1
+        val.add(checkNil(idgetter))
+        lastNum = id + 1
+      elif how[i + 1] in StartIdent:
+        i += 1
+        var name {.inject.} = ""
+        while i < how.len and how[i] in Ident:
+          name.add(how[i])
+          i += 1
+        val.add(checkNil(namegetter))
+      elif how[i + 1] == '{':
+        i += 2
+        var name {.inject.} = ""
+        while i < how.len and how[i] != '}':
+          name.add(how[i])
+          i += 1
+        i += 1
+        val.add(checkNil(namegetter))
+      else:
+        raise newException(Exception, "Syntax error in format string at " & $i)
   val

test/replace.nim

@@ -14,3 +14,7 @@ suite "replace":
     check("123".replace(re"(\d)(\d)", "$#$#") == "123")
     check("123".replace(re"(?<foo>\d)(\d)", "$foo$#$#") == "1123")
     check("123".replace(re"(?<foo>\d)(\d)", "${foo}$#$#") == "1123")
+
+  test "replacing missing captures should throw instead of segfaulting":
+    expect ValueError: discard "ab".replace(re"(a)|(b)", "$1$2")
+    expect ValueError: discard "b".replace(re"(a)?(b)", "$1$2")