You can't do that. Otherwise it would be very easy for any application to get complete control over the CPU timeslices assigned to it.
You can, however, give your process a high priority to reduce the probability of a context-switch.
Here is another thought:
Assuming that you don't measure the execution time of a regular expression just once but multiple times, you should not see the average execution time as an absolute value but as a relative value compared to the average execution times of other regular expressions.
With this thinking you can compare the average execution times of different regular expressions without knowing the times lost to context switches. The time lost to context switches would be about the same in every average, assuming the environment is relatively stable with regards to CPU utilization.