regex - What is '?-mix' in a Ruby Regular Expression

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Just trying to debug a regular
expression in ruby. When I print the contents of a regular expression, it shows
?-mix at the beginning of the regular expression even though
those characters were not part of the expression. Please see the following IRB output to
see this
illustrated

irb(main):028:0* EXPR
            = /^a$/
=> /^a$/

irb(main):029:0>
            EXPR
=> /^a$/
irb(main):030:0> puts
            EXPR
(?-mix:^a$)
=>
            nil

As you can see,
when you use puts to print out the contents of a regular
expression, there is ?-mix at the beginning. Should I be
concerned by this? Where is it coming from?

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class="normal">Answer

mix is not the
English word mix, it's options of
Regexp.

See
href="http://ruby-doc.org/core-2.2.0/Regexp.html#method-i-to_s">Regexp#to_s:

Returns a string containing the regular expression and its options (using the
(?opts:source) notation.

In your example,
m is for multiline mode,
i is for case insensitive, and
x is for extended mode.
Options before the dash are on, those after are off
(default). The question's example, ?-mix, has
all options off.

You can turn them on
like:

puts
            /^a$/mix
# =>(?mix:^a$)