G2 does not support accented entries in the alphabar

Problem/Motivation

The g2.initial route used to only accept ASCII letters, decimal digits, and space, dash and underscore are valid initials. This has been extended to a non-english-centric view, using Unicode letter and number classes. However, the alphabar throws exceptions when these are included.

Steps to reproduce

• Define a G2 entry like "été"
• Add é to the alphabar
• Navigate to /g2
• Observe the exception

Proposed resolution

The underlying issue is core bug #3369925: Support Unicode regular expressions in routes, from Symfony 4.3: the URL generator does not match requirements using full Unicode matching.

Remaining tasks

Get a fix in core for the bug.

Until then, keep using the restricted legacy regexp, and avoid starting entries with non-ASCII numbers and letters.

User interface changes

None.

API changes

None.

Data model changes

None.