The Regular Expression Engine branch under Configuration - Search has the following options:
Perl Compatible Regular Expressions
If this option is selected the editor will use Perl style regular expressions based on the Boost C++ Libraries when the Regular Expressions option is selected in the Find/Replace dialogs.
UltraEdit Style Regular Expressions
If this option is selected the editor will use UltraEdit style regular expressions when the Regular Expressions option is selected in the Find/Replace dialogs.
UNIX Style Regular Expressions
If this option is selected the editor will use UNIX style regular expressions when the Regular Expressions option is selected in the Find/Replace dialogs.