What's the difference between /\p{Alpha}/i and /\p{L}/i in ruby?

regex, ruby

Solution

They seem to be equivalent. (Edit: sometimes, see the end of this answer)

It seems like Ruby supports `\p{Alpha}` since version 1.9. In POSIX `\p{Alpha}` is equal to `\p{L&}` (for regular expressions with Unicode support; see here). This matches all characters that have an upper and lower case variant (see here). Unicase letters would not be matched (while they would be match by `\p{L}`.

This does not seem to be true for Ruby (I picked a random Arabic character, since Arabic has a unicase alphabet):

- `\p{L}` (any letter) matches.

- Case-sensitive classes `\p{Lu}`, `\p{Ll}`, `\p{Lt}` don't match. As expected.

- `p{L&}` doesn't match. As expected.

- `\p{Alpha}` matches.

Which seems to be a very good indication that `\p{Alpha}` is just an alias for `\p{L}` in Ruby. On Rubular you can also see that `\p{Alpha}` was not available in Ruby 1.8.7.

Note that the `i` modifier is irrelevant in any case, because both `\p{Alpha}` and `\p{L}` match both upper- and lower-case characters anyway.

EDIT:

A ha, there is a difference! I just found this PDF about Ruby's new regex engine (in use as of Ruby 1.9 as stated above). `\p{Alpha}` is available regardless of encoding (and will probably just match `[A-Za-z]` if there is no Unicode support), while `\p{L}` is specifically a Unicode property. That means, `\p{Alpha}` behaves exactly as in POSIX regexes, with the difference that here is corresponds to `\p{L}`, but in POSIX it corresponds to `\p{L&}`.

Problem

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