reLoop function
Create a regular expression loop. The supplied regular expression expr is
repeated between low and high times. The low should be below high
with one exception: when supplying the value high as 0, the meaning is to
repeat the argument expr at least low number of times, and with an
unbounded upper bound.
Implementation
ReLoop reLoop(Expr expr, int low, int high) =>
ReLoop(expr, low, high).declare();