Acutally a regex isn't all that good of an idea. It's very slow. But substr is better than chop.. benchmarks included..

Benchmark: timing 100000 iterations of chop, math, regex, substr...
      chop:  6 wallclock secs ( 4.92 usr +  0.03 sys = 4.95 CPU) @ 202
+02.02/s (n=100000)
      math:  2 wallclock secs ( 1.07 usr +  0.02 sys = 1.09 CPU) @ 917
+43.12/s (n=100000)
     regex: 25 wallclock secs (17.40 usr +  0.32 sys = 17.72 CPU) @ 56
+43.34/s (n=100000)
    substr:  2 wallclock secs ( 1.99 usr +  0.03 sys = 2.02 CPU) @ 495
+04.95/s (n=100000)
[download]

Doing the math is still the best option..