Hello:
I’m not sure whether it’s gnu sed’s bug, but it just behaves  different with Mac OS X :

gnu sed(/usr/local/bin/sed (GNU sed) 4.2.2):
  1. ~ $ echo '<WebCore/DOMElement.h>'| /usr/local/bin/sed -E 's/\<WebCore\//\<WebKitLegacy\//'
  2. <<WebKitLegacy/DOMElement.h>

Mac 10.11.3 sed(/usr/bin/sed):
  1. ~ $ echo '<WebCore/DOMElement.h>'| /usr/bin/sed -E 's/\<WebCore\//\<WebKitLegacy\//'
  2. <WebKitLegacy/DOMElement.h>