fixes #1724 for heregexen

TODO: DRY up that regex handling code so we don't have a duplicate test/error

lib/coffee-script/lexer.js

@@ -207,6 +207,9 @@
       heregex = match[0], body = match[1], flags = match[2];
       if (0 > body.indexOf('#{')) {
         re = body.replace(HEREGEX_OMIT, '').replace(/\//g, '\\/');
+        if (re.match(/^\*/)) {
+          this.error('regular expressions cannot begin with `*`');
+        }
         this.token('REGEX', "/" + (re || '(?:)') + "/" + flags);
         return heregex.length;
       }

src/lexer.coffee

@@ -209,6 +209,7 @@ exports.Lexer = class Lexer
     [heregex, body, flags] = match
     if 0 > body.indexOf '#{'
       re = body.replace(HEREGEX_OMIT, '').replace(/\//g, '\\/')
+      if re.match /^\*/ then @error 'regular expressions cannot begin with `*`'
       @token 'REGEX', "/#{ re or '(?:)' }/#{flags}"
       return heregex.length
     @token 'IDENTIFIER', 'RegExp'

test/regexps.coffee

@@ -52,3 +52,6 @@ test "a heregex will ignore whitespace and comments", ->
 
 test "an empty heregex will compile to an empty, non-capturing group", ->
   eq /(?:)/ + '', ///  /// + ''
+
+test "#1724: regular expressions beginning with `*`", ->
+  throws -> CoffeeScript.compile '/// * ///'