[ruby-core:108948] [Ruby master Feature#18788] Support passing Regexp options as String to Regexp.new

Issue #18788 has been updated by matz (Yukihiro Matsumoto).


Accepted. Unknown flags should raise errors.

Matz.


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Feature #18788: Support passing Regexp options as String to Regexp.new
https://bugs.ruby-lang.org/issues/18788#change-98043

* Author: janosch-x (Janosch M端ller)
* Status: Open
* Priority: Normal
----------------------------------------
## Current situation

`Regexp.new` takes an integer as second argument which needs to be ORed together from multiple constants:

```ruby
Regexp.new('foo', Regexp::IGNORECASE | Regexp::MULTILINE | Regexp::EXTENDED) # => /foo/imx
```

Any other non-nil value is treated as `i` flag:

```ruby
Regexp.new('foo', Object.new) # => /foo/i
```

## Suggestion

`Regexp.new` should support passing the regexp flags not only as an Integer, but also as a String, like so:

```ruby
Regexp.new('foo', 'i')   # => /foo/i
Regexp.new('foo', 'imx') # => /foo/imx

# edge cases
Regexp.new('foo', 'iii') # => /foo/i
Regexp.new('foo', '')    # => /foo/

# unsupported flags should probably emit a warning
Regexp.new('foo', 'jmq') # => /foo/m
Regexp.new('foo', '-m')  # => /foo/m
```

## Reasons

1. The constants are a bit cumbersome to use, particularly when building the regexp from variable data:

```ruby
def make_regexp(regexp_body, opt_string)
  opt_int = 0
  opt_int |= Regexp::IGNORECASE if opt_string.include?('i')
  opt_int |= Regexp::MULTILINE  if opt_string.include?('m')
  opt_int |= Regexp::EXTENDED   if opt_string.include?('x')

  Regexp.new(regexp_body, opt_int)
end
```

2. Passing a String is already silently accepted, and people might get the wrong impression that it works:

```ruby
Regexp.new('foo', 'i') # => /foo/i
```

... but it doesn't really work:

```ruby
Regexp.new('foo', 'x') # => /foo/i
```

## Backwards compatibility

This change would not be fully backwards compatible.

Code that relies on the second argument being a String which does not contain "i" in order to make the Regexp case insensitive would break.

*Note: originally I suggested supporting Symbols in the same way as Strings, but removed that in light of the discussion.*




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