empty regular expressions with flags still need to be compiled to /(?:)/

lib/coffee-script/lexer.js

@@ -181,7 +181,7 @@
     };
 
     Lexer.prototype.regexToken = function() {
-      var length, match, prev, regex, _ref3;
+      var flags, length, match, prev, regex, _ref3, _ref4;
       if (this.chunk.charAt(0) !== '/') return 0;
       if (match = HEREGEX.exec(this.chunk)) {
         length = this.heregexToken(match);
@@ -193,13 +193,13 @@
         return 0;
       }
       if (!(match = REGEX.exec(this.chunk))) return 0;
-      regex = match[0];
-      if (regex.match(/^\/\*/)) {
+      _ref4 = match, match = _ref4[0], regex = _ref4[1], flags = _ref4[2];
+      if (regex.slice(0, 2) === '/*') {
         this.error('regular expressions cannot begin with `*`');
       }
       if (regex === '//') regex = '/(?:)/';
-      this.token('REGEX', regex);
-      return regex.length;
+      this.token('REGEX', "" + regex + flags);
+      return match.length;
     };
 
     Lexer.prototype.heregexToken = function(match) {
@@ -638,7 +638,7 @@
 
   JSTOKEN = /^`[^\\`]*(?:\\.[^\\`]*)*`/;
 
-  REGEX = /^\/(?![\s=])[^[\/\n\\]*(?:(?:\\[\s\S]|\[[^\]\n\\]*(?:\\[\s\S][^\]\n\\]*)*])[^[\/\n\\]*)*\/[imgy]{0,4}(?!\w)/;
+  REGEX = /^(\/(?![\s=])[^[\/\n\\]*(?:(?:\\[\s\S]|\[[^\]\n\\]*(?:\\[\s\S][^\]\n\\]*)*])[^[\/\n\\]*)*\/)([imgy]{0,4})(?!\w)/;
 
   HEREGEX = /^\/{3}([\s\S]+?)\/{3}([imgy]{0,4})(?!\w)/;
 

src/lexer.coffee

@@ -198,11 +198,11 @@ exports.Lexer = class Lexer
     prev = last @tokens
     return 0 if prev and (prev[0] in (if prev.spaced then NOT_REGEX else NOT_SPACED_REGEX))
     return 0 unless match = REGEX.exec @chunk
-    [regex] = match
-    if regex.match /^\/\*/ then @error 'regular expressions cannot begin with `*`'
+    [match, regex, flags] = match
+    if regex[..1] is '/*' then @error 'regular expressions cannot begin with `*`'
     if regex is '//' then regex = '/(?:)/'
-    @token 'REGEX', regex
-    regex.length
+    @token 'REGEX', "#{regex}#{flags}"
+    match.length
 
   # Matches multiline extended regular expressions.
   heregexToken: (match) ->
@@ -607,7 +607,7 @@ JSTOKEN    = /^`[^\\`]*(?:\\.[^\\`]*)*`/
 
 # Regex-matching-regexes.
 REGEX = /// ^
-  / (?! [\s=] )       # disallow leading whitespace or equals signs
+  (/ (?! [\s=] )   # disallow leading whitespace or equals signs
   [^ [ / \n \\ ]*  # every other thing
   (?:
     (?: \\[\s\S]   # anything escaped
@@ -617,7 +617,7 @@ REGEX = /// ^
          ]
     ) [^ [ / \n \\ ]*
   )*
-  / [imgy]{0,4} (?!\w)
+  /) ([imgy]{0,4}) (?!\w)
 ///
 
 HEREGEX      = /// ^ /{3} ([\s\S]+?) /{3} ([imgy]{0,4}) (?!\w) ///

test/regexps.coffee

@@ -55,3 +55,9 @@ test "an empty heregex will compile to an empty, non-capturing group", ->
 
 test "#1724: regular expressions beginning with `*`", ->
   throws -> CoffeeScript.compile '/// * ///'
+
+test "empty regular expressions with flags", ->
+  fn = (x) -> x
+  a = "" + //i
+  fn ""
+  eq '/(?:)/i', a