Ruby regex fails when constructed from a stored string

Choose single-quoted strings rather than double-quoted ones (see this reference (https://en.wikibooks.org/wiki/Ruby_Programming/Strings#Single_quotes))

> puts "/\A([^@\s]+)@((?:c-d+\.)+(in|com))\z/"
/A([^@ ]+)@((?:c-d+.)+(in|com))z/

> puts '/\A([^@\s]+)@((?:c-d+\.)+(in|com))\z/'
/\A([^@\s]+)@((?:c-d+\.)+(in|com))\z/

After that, discard the '/' at either end:

> pattern_text = '\A([^@\s]+)@((?:c-d+\.)+(in|com))\z'
=> "\\A([^@\\s]+)@((?:c-d+\\.)+(in|com))\\z"

> compiled = Regexp.new(pattern_text)
=> /\A([^@\s]+)@((?:c-d+\.)+(in|com))\z/

> "mailbox@c-d.in".match?(Regexp.new(compiled))
=> true

When putting a regular expression pattern into a database for later use, it is customary to save a string representation of the pattern / flags.

So, when all regex information belongs in one column, Regexp#to_s (https://ruby-doc.org/core-2.5.1/Regexp.html#method-i-to_s) is an option:

stored_expression = /.../.to_s

To keep the pattern (source (https://ruby-doc.org/core-2.5.1/Regexp.html#method-i-source)) separate from its flags (options (https://ruby-doc.org/core-2.5.1/Regexp.html#method-i-options)) as strings:

stored_pattern = /\A([^@\s]+)@((?:c-d+\.)+(in|com))\z/.source
stored_flags   = /\A([^@\s]+)@((?:c-d+\.)+(in|com))\z/.options

After you retreive those database values, reconstruct the regex object with the Regexp.new constructor (https://ruby-doc.org/core-2.5.1/Regexp.html#method-c-new):

expression = Regexp.new(stored_expression)
# alternatively
expression = Regexp.new(stored_pattern, stored_flags)

Consult the Ruby documentation for Regexp (https://ruby-doc.org/core-2.5.1/Regexp.html).