diff --git a/ysh/regex_translate.py b/ysh/regex_translate.py
index 9c55a85a70..4a97a5f647 100644
--- a/ysh/regex_translate.py
+++ b/ysh/regex_translate.py
@@ -14,10 +14,12 @@
     re_repeat,
     re_repeat_e,
     NameType,
+    EggexFlag,
 )
 from _devbuild.gen.id_kind_asdl import Id
 from _devbuild.gen.value_asdl import value
 from core.error import e_die
+from frontend import lexer
 from mycpp.mylib import log, tagswitch
 from osh import glob_  # for ExtendedRegexEscape
 
@@ -346,3 +348,25 @@ def AsPosixEre(eggex):
     eggex.name_types = name_types
 
     return eggex.as_ere
+
+
+def _EncodeFlags(flags):
+    # type: (List[EggexFlag]) -> str
+    """
+    Raises fatal exception on invalid flags.
+    """
+    letters = []  # type: List[str]
+    for flag in flags:
+        if flag.negated:
+            e_die("Flag can't be negated", flag.flag)
+        flag_name = lexer.TokenVal(flag.flag)
+        if flag_name in ('i', 'reg_icase'):
+            letters.append('i')
+        elif flag_name == 'reg_newline':
+            letters.append('n')
+        else:
+            e_die("Invalid regex flag %r" % flag_name, flag.flag)
+
+    # Normalize for comparison
+    return ''.join(sorted(letters))
+
diff --git a/ysh/regex_translate_test.py b/ysh/regex_translate_test.py
new file mode 100755
index 0000000000..c48d501149
--- /dev/null
+++ b/ysh/regex_translate_test.py
@@ -0,0 +1,51 @@
+#!/usr/bin/env python2
+from __future__ import print_function
+
+import unittest
+
+from _devbuild.gen.id_kind_asdl import Id
+from _devbuild.gen.syntax_asdl import EggexFlag, Token, source, SourceLine
+
+from asdl import runtime
+from ysh import regex_translate
+
+
+def _Name(s):
+    # Doesn't work
+    #return lexer.DummyToken(Id.Expr_Name, s)
+    src = source.Stdin('')
+    source_line = SourceLine(1, s, src)
+    return Token(Id.Expr_Name, 0, len(s), runtime.NO_SPID, source_line, None)
+
+
+class RegexTranslateTest(unittest.TestCase):
+
+    def testEncodeFlags(self):
+        reg_icase = _Name('reg_icase')
+        i = _Name('i')  # abbreviation
+        reg_newline = _Name('reg_newline')
+        bad = _Name('bad')
+
+        flags = [EggexFlag(False, reg_icase)]
+        self.assertEqual('i', regex_translate._EncodeFlags(flags))
+
+        flags = [EggexFlag(False, i)]
+        self.assertEqual('i', regex_translate._EncodeFlags(flags))
+
+        flags = [EggexFlag(False, bad)]
+        try:
+            regex_translate._EncodeFlags(flags)
+        except Exception as e:
+            print(e.UserErrorString())
+        else:
+            self.fail('Should have failed')
+
+        order1 = [EggexFlag(False, reg_icase), EggexFlag(False, reg_newline)]
+        order2 = [EggexFlag(False, reg_newline), EggexFlag(False, reg_icase)]
+
+        self.assertEqual('in', regex_translate._EncodeFlags(order1))
+        self.assertEqual('in', regex_translate._EncodeFlags(order2))
+
+
+if __name__ == '__main__':
+    unittest.main()