--- a/mercurial/commands.py	Wed Jul 11 19:56:16 2007 -0300
+++ b/mercurial/commands.py	Mon Jul 09 17:41:14 2007 +0300
@@ -1092,7 +1092,11 @@
     reflags = 0
     if opts['ignore_case']:
         reflags |= re.I
-    regexp = re.compile(pattern, reflags)
+    try:
+        regexp = re.compile(pattern, reflags)
+    except Exception, inst:
+        ui.warn(_("grep: invalid match pattern: %s!\n") % inst)
+        return None
     sep, eol = ':', '\n'
     if opts['print0']:
         sep = eol = '\0'

--- a/tests/test-grep	Wed Jul 11 19:56:16 2007 -0300
+++ b/tests/test-grep	Mon Jul 09 17:41:14 2007 +0300
@@ -17,6 +17,8 @@
 head -n 3 port > port1
 mv port1 port
 hg commit -m 4 -u spam -d '4 0'
+echo % pattern error
+hg grep '**test**'
 echo % simple
 hg grep port port
 echo % all

--- a/tests/test-grep.out	Wed Jul 11 19:56:16 2007 -0300
+++ b/tests/test-grep.out	Mon Jul 09 17:41:14 2007 +0300
@@ -1,3 +1,5 @@
+% pattern error
+grep: invalid match pattern: nothing to repeat!
 % simple
 port:4:export
 port:4:vaportight