Currently an optional variable (that has a default and can be left out from matching and generating URLs) can only be specified without any text suffix, i.e. at the end of the pattern. This is quite limiting.

@laszlokorte and I propose to add the possibility to mark the optional parts with parentheses/curly brackets.
This can be implemented BC and would allow several new, but common use-cases, e.g.:

• one can specify optional variables anywhere in the pattern (not only at the end),
• one can use more than one char as seperator,
• an optional part that depends on multiple variables,
• flexiblity of where, what and whether there is a (trailing) seperator (see Consider adding the trailing separator as optional for optional values #5918).

Examples:

• /products(-by-{criterion}).html would either match /products.html with criterion = default or /products-by-price.html with criterion = 'price'
• /products(/{category}(.{_format})) would be equivalent to how we already handle /products/{category}.{_format} with defaults for both variables. It matches /products, /products/shoes and /products/shoes.html. But it does not match /products.html! Compare that to
• /products((/{category}).{_format}) (only brackets changed) where the format would match without category, but the category does not match without the format. So the inner part requires the outer part to match.
• /products(-in-{category})(-by-{criterion}).html matches /products.html and /products-in-shoes-by-prize.html and /products-in-shoes.html and /products-by-prize.html.
• /products(-in-{category1}-and-{category2}).html only matches the optional part when both variables match, so /products-in-shoes-and-suits.html and /products.html, but not /products-in-shoes.html.
• /products(-by-price).html should either raise exception that there is no variable in it, or treat it as-is (with the parentheses). It must not match both /products.html and /products-by-prize.html (where you should usually specify 2 routes for it) because there would be no way to decide which URL to generate.
• /products/({category}) would match /products/ and /products/... whereas without the parentheses it would match /products and /products/... (see Routes of the form /hello/{name} with an optional parameter at the end don't match as expected #5869)
• /{product}((-by-{criterion}).{_format}) would need a default requirement for product to be [^/-\.]++ (so as seperator both / (standard) and - and .. Pretty hard to compile.

Also, it should raise an exception when you specify an optional part explicitly without adding a default for the variable in it.

This feature request is somewhat related to #5129 and also covers #5869 and #10217 and #9981