Expand RegularFormula class to handle special characters via escape

One usage of RegularFormula class is to act as an expression used for filtering events based on the trigger decision, before the regular reconstruction runs, via the TriggerSelectorAlg. The current implementation however fails in case the trigger name contains a special character like -.

This MR fixes this problem allowing the input of this RegularFormula class to contain such characters if escaped via \, such that algorithms as TriggerSelectorAlg that use this class can now select correctly events based on triggers containing e.g. hyphens (-) in their name, when escaped properly.

Tagging @cohm