Matching a sliced string - ruby

Question

I am trying to use the case statement:
week # => "03 – 09 MAR 2019"
first_day = week.slice(0..2) # => 03
last_day = week.slice(5..7) # => 09
month = week.slice(8..11) # => MAR
year = week.slice(12..17) # => 2019
puts month # >> MAR
case month
when 'JAN' then month_num = '01'
when 'FEB' then month_num = '02'
when 'MAR' then month_num = '03'
when 'APR' then month_num = '04'
when 'MAY' then month_num = '05'
when 'JUN' then month_num = '06'
when 'JUL' then month_num = '07'
when 'AGO' then month_num = '08'
when 'SEP' then month_num = '09'
when 'OCT' then month_num = '10'
when 'NOV' then month_num = '11'
when 'DEC' then month_num = '12'
else month_num = 'ERROR'
end
puts month_num # >> ERROR
However, the case statement always goes to the else branch.
Why is the var month_num equal to the string "ERROR" instead of "03"?

Answer 1

You are using puts to examine what you have, and therefore you are missing to observe whitespaces in your results. You actually have:
week.slice(0..2) # => "03 "
week.slice(5..7) # => "09 "
week.slice(8..11) # => "MAR "
week.slice(12..17) # => "2019"
To observe what you have, it is better to use p rather than puts.
You have the wrong range. Actually, there is no reason to use ranges here. It is much easier to use the second argument to specify the length:
week.slice(0, 2) # => "03"
week.slice(5, 2) # => "09"
week.slice(8, 3) # => "MAR"
week.slice(12, 4) # => "2019"

Answer 2

Your month is "MAR "
Try
month = week.slice(8..10)
And makes sense, from 8 to 10 inclusive are three characters. Same for the other parts.

Answer 3

Seems like you want to parse a string containing data in a specific format. Instead of relying on absolute indices, you could use a regular expression to match the date format, e.g:
PATTERN = /
(?<first_day>\d{2}) # 2-digit first day
\s* # optional whitespace
[–-] # delimiter(s)
\s*
(?<last_day>\d{2}) # 2-digit last day
\s*
(?<month>\w{3}) # 3-letter month name
\s*
(?<year>\d{4}) # 4-digit year
/ix
To extract the data:
str = '03 – 09 MAR 2019'
m = str.match(PATTERN)
#=> #<MatchData "03 – 09 MAR 2019" first_day:"03" last_day:"09" month:"MAR" year:"2019">
m[:first_day] #=> "03"
m[:last_day] #=> "09"
m[:month] #=> "MAR"
m[:year] #=> "2019"
The results could further be fed into Date.strptime:
require 'date'
from = m.values_at(:first_day, :month, :year).join(' ') #=> "03 MAR 2019"
to = m.values_at(:first_day, :month, :year).join(' ') #=> "09 MAR 2019"
Date.strptime(from, '%d %b %Y') #=> #<Date: 2019-03-03 ...>
Date.strptime(to, '%d %b %Y') #=> #<Date: 2019-03-09 ...>
Or _strptime if you're just interested in the raw values:
Date._strptime(from, '%d %b %Y')
#=> {:mday=>3, :mon=>3, :year=>2019}