Use a regex to match which is stored in a hash - ruby

Question

Here is a Ruby code that explains the issue:
1.8.7 :018 > pattern[:key] = '554 5.7.1.*Service unavailable; Helo command .* blocked using'
=> "554 5.7.1.*Service unavailable; Helo command .* blocked using"
1.8.7 :019 > line = '554 5.7.1 Service unavailable; Helo command [abc] blocked using dbl'
=> "554 5.7.1 Service unavailable; Helo command [abc] blocked using dbl"
1.8.7 :020 > line =~ /554 5.7.1.*Service unavailable; Helo command .* blocked using/
=> 0
1.8.7 :021 > line =~ /pattern[:key]/
=> nil
1.8.7 :022 >
Regex works when using directly as a string but doesn't work when I'm using it as a value of hash.

Answer 1

This isn't a Ruby question per se, it's how to construct a regex pattern that accomplishes what you want.
In "regex-ese", /pattern[:key]/ means:
Find pattern.
Following pattern look for one of :, k, e or y.
Ruby doesn't automatically interpolate variables in strings or regex patterns like Perl does, so, instead, we have to mark where the variable is using #{...} inline.
If you're only using /pattern[:key]/ as a pattern, don't bother interpolating it into a pattern. Instead, take the direct path and let Regexp do it for you:
pattern[:key] = 'foo'
Regexp.new(pattern[:key])
=> /foo/
Which is the same result as:
/#{pattern[:key]}/
=> /foo/
but doesn't waste CPU cycles.
Another of your attempts used ., [ and ], which are reserved characters in patterns, used to help define patterns. If you need to use such characters, you can have Ruby's Regexp.escape add \ escape characters appropriately, preserving their normal/literal meaning in the string:
Regexp.escape('5.7.1 [abc]')
=> "5\\.7\\.1\\ \\[abc\\]"
which, in real life is "5\.7\.1\ \[abc\]" (when not being displayed in IRB)
To use that in a regex, use:
Regexp.new(Regexp.escape('5.7.1 [abc]'))
=> /5\.7\.1\ \[abc\]/

Answer 2

line =~ /#{pattern[:key]}/
or...
line =~ Regexp.new pattern[:key]
If you want to escape regex special characters...
line =~ /#{Regexp.quote pattern[:key]}/
Edit: Since you're new to ruby, may I suggest you do this, wherever pattern is defined:
pattern[:key] = Regexp.new '554 5.7.1.*Service unavailable; Helo command .* blocked using'
Then you can simple use the Regexp object stored in pattern
line =~ pattern[:key]