[rules-users] Contains substring in the LHS with regular expression capturing groups?

Hi, I need to check in the LHS of my rule if a given String contains an given substring. What is the best way to do that? I can use regular expression with matches, but is that the best (fastest) solution? (STRING matches "(?i).*SUBSTRING.*"). And can I use regulare expressions capturing groups and than use the results of the groups in the RHS? If not, what is the best methode to do that? Thanks, Waruzjan