How to build a case-insensitive regular expression with Regexp.union

Question

I have a list of strings, and need to build the regular expression from them, using Regexp#union. I need the resulting pattern to be case insensitive.

The #union method itself does not accept options/modifiers, hence I currently see two options:

strings = %w|one two three|

Regexp.new(Regexp.union(strings).to_s, true)

and/or:

Regexp.union(*strings.map { |s| /#{s}/i })

Both variants look a bit weird.

Is there an ability to construct a case-insensitive regular expression by using Regexp.union?

Answer 1

The simple starting place is:

words = %w[one two three]
/#{ Regexp.union(words).source }/i # => /one|two|three/i

You probably want to make sure you're only matching words so tweak it to:

/\b#{ Regexp.union(words).source }\b/i # => /\bone|two|three\b/i

For cleanliness and clarity I prefer using a non-capturing group:

/\b(?:#{ Regexp.union(words).source })\b/i # => /\b(?:one|two|three)\b/i

Using source is important. When you create a Regexp object, it has an idea of the flags (i, m, x) that apply to that object and those get interpolated into the string:

"#{ /foo/i }" # => "(?i-mx:foo)"
"#{ /foo/ix }" # => "(?ix-m:foo)"
"#{ /foo/ixm }" # => "(?mix:foo)"

or

(/foo/i).to_s  # => "(?i-mx:foo)"
(/foo/ix).to_s  # => "(?ix-m:foo)"
(/foo/ixm).to_s  # => "(?mix:foo)"

That's fine when the generated pattern stands alone, but when it's being interpolated into a string to define other parts of the pattern the flags affect each sub-expression:

/\b(?:#{ Regexp.union(words) })\b/i # => /\b(?:(?-mix:one|two|three))\b/i

Dig into the Regexp documentation and you'll see that ?-mix turns off "ignore-case" inside (?-mix:one|two|three), even though the overall pattern is flagged with i, resulting in a pattern that doesn't do what you want, and is really hard to debug:

'foo ONE bar'[/\b(?:#{ Regexp.union(words) })\b/i] # => nil

Instead, source removes the inner expression's flags making the pattern do what you'd expect:

/\b(?:#{ Regexp.union(words).source })\b/i # => /\b(?:one|two|three)\b/i

and

'foo ONE bar'[/\b(?:#{ Regexp.union(words).source })\b/i] # => "ONE"

You can build your patterns using Regexp.new and passing in the flags:

regexp = Regexp.new('(?:one|two|three)', Regexp::EXTENDED | Regexp::IGNORECASE) # => /(?:one|two|three)/ix

but as the expression becomes more complex it becomes unwieldy. Building a pattern using string interpolation remains more easy to understand.