how to match this pattern using regex - ruby

Question

I am new to ruby and trying to use regular expression.
Basically I want to read a file and check if it has the right format.
Requirements to be in the correct format:
1: The word should start with from
2: There should be one space and only one space is allowed, unless there is a comma
3: Not consecutive commas
4: from and to are numbers
5: from and to must contain a colon
from: z to: 2
from: 1 to: 3,4
from: 2 to: 3
from:3 to: 5
from: 4 to: 5
from: 4 to: 7
to: 7 from: 6
from: 7 to: 5
0: 7 to: 5
from: 24 to: 5
from: 7 to: ,,,5
from: 8 to: 5,,5
from: 9 to: ,5
If I have the correct regular expression, then the output should be:
from: 1 to: 3,4
from: 2 to: 3
from: 4 to: 5
from: 4 to: 7
from: 7 to: 5
from: 24 to: 5
so in this case these are the false ones:
from: z to: 2 # because starts with z
from:3 to: 5 # because there is no space after from:
to: 7 from: 6 # because it starts with to but supposed to start with from
0: 7 to: 5 # starts with 0 instead of from
from: 7 to: ,,,5 # because there are two consecutive commas
from: 8 to: 5,,5 # two consecutive commas
from: 9 to: ,5 # start with comma

Answer 1

OK, the regex you want is something like this:
from: \d+(?:,\d+)* to: \d+(?:,\d+)*
This assumes that multiple numbers are permitted in the from: column as well. If not, you want this one:
from: \d+ to: \d+(?:,\d+)*
To verify that the whole file is valid (assuming all it contains are lines like this one), you could use a function like this:
def validFile(filename)
File.open(filename).each do |line|
return false if (!/\d+(?:,\d+)* to: \d+(?:,\d+)*/.match(line))
end
return true
end

Answer 2

What you are looking for is called negative lookahead. Specifically, \d+(?!,,) which says: match 1 or more consecutive digits not followed by 2 commas. Here is the whole thing:
str = "from: z to: 2
from: 1 to: 3,4
from: 2 to: 3
from:3 to: 5
from: 4 to: 5
from: 4 to: 7
to: 7 from: 6
from: 7 to: 5
0: 7 to: 5
from: 24 to: 5
from: 7 to: ,,,5
from: 8 to: 5,,5
from: 9 to: ,5
"
str.each_line do |line|
puts(line) if line =~ /\Afrom: \d+ to: \d+(?!,,)/
end
Output:
from: 1 to: 3,4
from: 2 to: 3
from: 4 to: 5
from: 4 to: 7
from: 7 to: 5
from: 24 to: 5