An explanation of the time regex found in the Saas Book.

Time_Regex(Ruby)

#!/usr/bin/env ruby
time_regex = /^\d\d?:\d\d\s*[ap]m$/i

#^ here means beginning of line
#\d is a digit, \s is a (white)space character

#the ? means 0 or 1 and affects the character before it. so here it means that
#the second digit is optional. Ie if someone wrote 9:25 as opposed to 09:25
#both would be understood. Similarly, the * following \s means 0+ and states
#that there could be any amount of spaces

#[] is a set, so here [ap] means that the character can be either an a or a p
#$ means end of line
# the i after the last / means that the whole regex is case-insensitive

#Alternate syntax would be:
x =~ /(\d\d?):(\d\d)\s*([ap])m$/i

#note that =~ returns the position of the matching text