diff --git a/lib/coffeescript/lexer.js b/lib/coffeescript/lexer.js
index b4618cbc..459b3db8 100644
--- a/lib/coffeescript/lexer.js
+++ b/lib/coffeescript/lexer.js
@@ -1,11 +1,11 @@
 // Generated by CoffeeScript 2.0.0-beta1
 (function() {
-  var BOM, BOOL, CALLABLE, CODE, COFFEE_ALIASES, COFFEE_ALIAS_MAP, COFFEE_KEYWORDS, COMMENT, COMPARE, COMPOUND_ASSIGN, HERECOMMENT_ILLEGAL, HEREDOC_DOUBLE, HEREDOC_INDENT, HEREDOC_SINGLE, HEREGEX, HEREGEX_OMIT, HERE_JSTOKEN, IDENTIFIER, INDENTABLE_CLOSERS, INDEXABLE, INVERSES, JSTOKEN, JS_KEYWORDS, LEADING_BLANK_LINE, LINE_BREAK, LINE_CONTINUER, Lexer, MATH, MULTI_DENT, NOT_REGEX, NUMBER, OPERATOR, POSSIBLY_DIVISION, REGEX, REGEX_FLAGS, REGEX_ILLEGAL, REGEX_INVALID_ESCAPE, RELATION, RESERVED, Rewriter, SHIFT, SIMPLE_STRING_OMIT, STRICT_PROSCRIBED, STRING_DOUBLE, STRING_INVALID_ESCAPE, STRING_OMIT, STRING_SINGLE, STRING_START, TRAILING_BLANK_LINE, TRAILING_SPACES, UNARY, UNARY_MATH, UNICODE_CODE_POINT_ESCAPE, VALID_FLAGS, WHITESPACE, compact, count, invertLiterate, isForFrom, isUnassignable, key, locationDataToString, repeat, starts, throwSyntaxError,
+  var BOM, BOOL, CALLABLE, CODE, COFFEE_ALIASES, COFFEE_ALIAS_MAP, COFFEE_KEYWORDS, COMMENT, COMPARE, COMPOUND_ASSIGN, HERECOMMENT_ILLEGAL, HEREDOC_DOUBLE, HEREDOC_INDENT, HEREDOC_SINGLE, HEREGEX, HEREGEX_OMIT, HERE_JSTOKEN, IDENTIFIER, INDENTABLE_CLOSERS, INDEXABLE, INVERSES, JSTOKEN, JS_KEYWORDS, LEADING_BLANK_LINE, LINE_BREAK, LINE_CONTINUER, Lexer, MATH, MULTI_DENT, NOT_REGEX, NUMBER, OPERATOR, POSSIBLY_DIVISION, REGEX, REGEX_FLAGS, REGEX_ILLEGAL, REGEX_INVALID_ESCAPE, RELATION, RESERVED, Rewriter, SHIFT, SIMPLE_STRING_OMIT, STRICT_PROSCRIBED, STRING_DOUBLE, STRING_INVALID_ESCAPE, STRING_OMIT, STRING_SINGLE, STRING_START, TRAILING_BLANK_LINE, TRAILING_SPACES, UNARY, UNARY_MATH, UNICODE_CODE_POINT_ESCAPE, VALID_FLAGS, WHITESPACE, compact, count, invertLiterate, isForFrom, isUnassignable, key, locationDataToString, merge, repeat, starts, throwSyntaxError,
     indexOf = [].indexOf;
 
   ({Rewriter, INVERSES} = require('./rewriter'));
 
-  ({count, starts, compact, repeat, invertLiterate, locationDataToString, throwSyntaxError} = require('./helpers'));
+  ({count, starts, compact, repeat, invertLiterate, merge, locationDataToString, throwSyntaxError} = require('./helpers'));
 
   exports.Lexer = Lexer = class Lexer {
     tokenize(code, opts = {}) {
@@ -366,9 +366,6 @@
             isRegex: true,
             offsetInChunk: 1
           });
-          body = this.formatRegex(body, {
-            delimiter: '/'
-          });
           index = regex.length;
           prev = this.prev();
           if (prev) {
@@ -398,8 +395,13 @@
           });
           break;
         case !(regex || tokens.length === 1):
-          if (body == null) {
-            body = this.formatHeregex(tokens[0][1]);
+          if (body) {
+            body = this.formatRegex(body, {
+              flags,
+              delimiter: '/'
+            });
+          } else {
+            body = this.formatHeregex(tokens[0][1], {flags});
           }
           this.token('REGEX', `${this.makeDelimitedLiteral(body, {
             delimiter: '/'
@@ -412,7 +414,9 @@
           this.mergeInterpolationTokens(tokens, {
             delimiter: '"',
             double: true
-          }, this.formatHeregex);
+          }, (str) => {
+            return this.formatHeregex(str, {flags});
+          });
           if (flags) {
             this.token(',', ',', index - 1, 0);
             this.token('STRING', '"' + flags + '"', index - 1, flags.length);
@@ -893,10 +897,10 @@
       return this.replaceUnicodeCodePointEscapes(str.replace(STRING_OMIT, '$1'), options);
     }
 
-    formatHeregex(str) {
-      return this.formatRegex(str.replace(HEREGEX_OMIT, '$1$2'), {
+    formatHeregex(str, options) {
+      return this.formatRegex(str.replace(HEREGEX_OMIT, '$1$2'), merge(options, {
         delimiter: '///'
-      });
+      }));
     }
 
     formatRegex(str, options) {
@@ -919,6 +923,8 @@
     }
 
     replaceUnicodeCodePointEscapes(str, options) {
+      var shouldReplace;
+      shouldReplace = (options.flags != null) && indexOf.call(options.flags, 'u') < 0;
       return str.replace(UNICODE_CODE_POINT_ESCAPE, (match, escapedBackslash, codePointHex, offset) => {
         var codePointDecimal;
         if (escapedBackslash) {
@@ -931,6 +937,9 @@
             length: codePointHex.length + 4
           });
         }
+        if (!shouldReplace) {
+          return match;
+        }
         return this.unicodeCodePointToUnicodeEscapes(codePointDecimal);
       });
     }
diff --git a/src/lexer.coffee b/src/lexer.coffee
index 26b302bf..51618e63 100644
--- a/src/lexer.coffee
+++ b/src/lexer.coffee
@@ -12,7 +12,7 @@
 {Rewriter, INVERSES} = require './rewriter'
 
 # Import the helpers we need.
-{count, starts, compact, repeat, invertLiterate,
+{count, starts, compact, repeat, invertLiterate, merge,
 locationDataToString,  throwSyntaxError} = require './helpers'
 
 # The Lexer Class
@@ -330,7 +330,6 @@ exports.Lexer = class Lexer
       when match = REGEX.exec @chunk
         [regex, body, closed] = match
         @validateEscapes body, isRegex: yes, offsetInChunk: 1
-        body = @formatRegex body, delimiter: '/'
         index = regex.length
         prev = @prev()
         if prev
@@ -349,13 +348,17 @@ exports.Lexer = class Lexer
       when not VALID_FLAGS.test flags
         @error "invalid regular expression flags #{flags}", offset: index, length: flags.length
       when regex or tokens.length is 1
-        body ?= @formatHeregex tokens[0][1]
+        if body
+          body = @formatRegex body, { flags, delimiter: '/' }
+        else
+          body = @formatHeregex tokens[0][1], { flags }
         @token 'REGEX', "#{@makeDelimitedLiteral body, delimiter: '/'}#{flags}", 0, end, origin
       else
         @token 'REGEX_START', '(', 0, 0, origin
         @token 'IDENTIFIER', 'RegExp', 0, 0
         @token 'CALL_START', '(', 0, 0
-        @mergeInterpolationTokens tokens, {delimiter: '"', double: yes}, @formatHeregex
+        @mergeInterpolationTokens tokens, {delimiter: '"', double: yes}, (str) =>
+          @formatHeregex str, { flags }
         if flags
           @token ',', ',', index - 1, 0
           @token 'STRING', '"' + flags + '"', index - 1, flags.length
@@ -792,8 +795,8 @@ exports.Lexer = class Lexer
   formatString: (str, options) ->
     @replaceUnicodeCodePointEscapes str.replace(STRING_OMIT, '$1'), options
 
-  formatHeregex: (str) ->
-    @formatRegex str.replace(HEREGEX_OMIT, '$1$2'), delimiter: '///'
+  formatHeregex: (str, options) ->
+    @formatRegex str.replace(HEREGEX_OMIT, '$1$2'), merge(options, delimiter: '///')
 
   formatRegex: (str, options) ->
     @replaceUnicodeCodePointEscapes str, options
@@ -808,8 +811,9 @@ exports.Lexer = class Lexer
     low = (codePoint - 0x10000) % 0x400 + 0xDC00
     "#{toUnicodeEscape(high)}#{toUnicodeEscape(low)}"
 
-  # Replace \u{...} with \uxxxx[\uxxxx] in strings and regexes
+  # Replace \u{...} with \uxxxx[\uxxxx] in regexes without `u` flag
   replaceUnicodeCodePointEscapes: (str, options) ->
+    shouldReplace = options.flags? and 'u' not in options.flags
     str.replace UNICODE_CODE_POINT_ESCAPE, (match, escapedBackslash, codePointHex, offset) =>
       return escapedBackslash if escapedBackslash
 
@@ -818,6 +822,7 @@ exports.Lexer = class Lexer
         @error "unicode code point escapes greater than \\u{10ffff} are not allowed",
           offset: offset + options.delimiter.length
           length: codePointHex.length + 4
+      return match unless shouldReplace
 
       @unicodeCodePointToUnicodeEscapes codePointDecimal
 
diff --git a/test/regexps.coffee b/test/regexps.coffee
index 0061a5a1..5ee91a15 100644
--- a/test/regexps.coffee
+++ b/test/regexps.coffee
@@ -305,12 +305,12 @@ test "#4248: Unicode code point escapes", ->
   ok ///a\u{000001ab}c///.test 'a\u{1ab}c'
   ok /a\u{12345}c/.test 'a\ud808\udf45c'
 
-  # rewrite code point escapes
+  # rewrite code point escapes unless u flag is set
   input = """
     /\\u{bcdef}\\u{abc}/u
     """
   output = """
-    /\\udab3\\uddef\\u0abc/u;
+    /\\u{bcdef}\\u{abc}/u;
   """
   eq toJS(input), output
 
diff --git a/test/strings.coffee b/test/strings.coffee
index 1cd17efa..a2bd8041 100644
--- a/test/strings.coffee
+++ b/test/strings.coffee
@@ -420,12 +420,12 @@ test "#4248: Unicode code point escapes", ->
   eq '\udab3\uddefc', """\u{bcdef}#{ 'c' }"""
   eq '\\u{123456}', "#{'\\'}#{'u{123456}'}"
 
-  # rewrite code point escapes
+  # don't rewrite code point escapes
   input = """
     '\\u{bcdef}\\u{abc}'
     """
   output = """
-    '\\udab3\\uddef\\u0abc';
+    '\\u{bcdef}\\u{abc}';
   """
   eq toJS(input), output
 
@@ -433,6 +433,6 @@ test "#4248: Unicode code point escapes", ->
     "#{ 'a' }\\u{bcdef}"
     """
   output = """
-    "a\\udab3\\uddef";
+    "a\\u{bcdef}";
   """
   eq toJS(input), output