Another way to do that is to turn x off for part of the pattern, either by toggling it, / ... (?-x)Mon Mar (?x) ... /x, or by localising the effect inside parentheses, / ... (?-x:Mon Mar ) ... /x. The technique can also be used for the i flag and, I think though I've never tried it, for s and m as well.

I hope this is of interest.

Cheers,

JohnGG