Not understanding regex (1[0-2]|0?[1-9]) for validating time strings

Question

I found and tested a regex to validate a time string such as 11:30 AM:

^(1[0-2]|0?[1-9]):([0-5][0-9])(\s[A|P]M)\)?$

I understand most of it except the beginning:

(1[0-2]|0?[1-9])

Can someone explain what is going on? 1[0-2] is there is a fist digit that can be between 0 and 2? And then I don't understand |0?[1-9].

Answer 1

(1[0-2]|0?[1-9])

| separates the regex into two parts, where

1[0-2]

matches 10, 11 or 12, and

0?[1-9]

matches 1 to 9, with an optional leading 0.

Answer 2

I will explain by writing the regex in extended mode, which permits comments:

r = /
    ^     # match the beginning of the string
    (     # begin capture group 1
    1     # match 1
    [0-2] # match one of the characters 0,1,2
    |     # or
    0?    # optionally match a zero
    [1-9] # match one of the characters between 1 and 9
    )     # end capture group 1
    :     # match a colon
    (     # begin capture group 2
    [0-5] # match one of the characters between 0 and 5
    [0-9] # match one of the characters between 0 and 9
    )     # end capture group 2
    (     # begin capture group 3
    \s    # match one whitespace character
    [A|P] # match one of the characters A, | or P
    M     # match M
    )     # end capture group 3
    \)?   # optionally match a right parenthesis
    $     # match the end of the string
    /x    # extended mode

As noticed by @Mischa, [A|P] is incorrect. It should be [AP]. That's because "|" is just an ordinary character when it's within a character class.

Also, I think the regex would be improved by moving \s out of capture group 3. We therefore might write:

r = /^(1[0-2]|0?[1-9]):([0-5][0-9])\s([AP]M)\)?$/

It could be used thusly:

result = "11:39 PM" =~ r
if result
  puts "It's #{$2} minutes past #{$1}, #{ $3=='AM' ? 'anti' : 'post' } meridiem."
else
  # raise exception?
end
  #=> It's 39 minutes past 11, post meridiem.

In words, the revised regex reads as follows:

• match the beginning of the string.
• match "10", "11", "12", or one of the digits "1" to "9", optionally preceded by a zero, and save the match to capture group 1.
• match a colon.
• match a digit between "0" and "5", then a digit between "0" and "9", and save the two digits to capture group 2.
• match a whitespace character.
• match "A", or "P", followed by "M", and save the two characters to capture group 3.
• optionally match a right parenthesis.
• match the end of the string.