Property Validation: `pattern`

[everettraven]

The pattern constraint on a property puts a restriction on the value of a property based on a regular expression.

It is non-trivial to determine whether or not a change to a regular expression is more/less restrictive via code, but generally we should at the very least flag changes to the pattern constraint so that a human-in-the-loop can evaluate and determine whether or not the change is safe to make.

More nuanced:

• Adding a pattern when one did not previously exist is a breaking change.
• Changing a pattern could be a breaking change.
• Removing a pattern is less restrictive and likely an OK change to make.

[LoginovIlia]

/assign

[LoginovIlia]

@everettraven
Hi. The changes have been merged, but I forgot to add "Fixes" before merging, so this task can now be closed.