Any replacements on word boundary \< \> will hang if the s/// is a non-word boundary char i.e. [^a-zA-Z0-9_]
echo foo | sed 's/\</a/g'
echo foo | sed 's/\</#/g'
Workaround is to use /usr/xpg4/bin/sed.
Bug # 7107401 has been logged for this kind of behavior with sed command.