safe regexp escape.md

Suppose we have a RegExp.escape which escapes:

• every ASCII punctuator except _, i.e. (){}[]|,.?*+-^$=<>\/#&!%:;@~'"`.
• whitespace
• 0-9 if at the start of the string (with a hex escape)

And we make \- and other currently-illegal escape sequences which would be produced by this function legal in u/v-mode RegExps (to mean the unescaped char), including inside of character classes.

And you don't put the output in a place where it would obviously mean something else, i.e. not

• immediately after \x, \x0, \u00, \c, etc
• immediately after an odd number of backslashes
• in (?${here}:asdf) (because of regexp modifiers)

Then escape is safe, i.e. it cannot lead to context escapes.

Specifically, we have the following contexts:

context	cannot leave context because
"base" context	trivial
character class	can't output unescaped ], ^, -, &, \ (etc)
(...) group	can't output unescaped ) or ?
\u{...}	can't output unescaped }
\k<...>	can't output unescaped >
(?<...>)	can't output unescaped >
foo{...}	can't output unescaped } or ,
\p{...}	can't output unescaped } or =
\q{...}	can't output unescaped } or |
after \1	numbers at the start of strings are escaped

And the following proposed future contexts:

context	cannot leave context because
(?#...)	can't output unescaped )
#... line comments	can't output unescaped line terminator
x-mode regexps	can't output unescaped whitespace
(?(...)...) conditions	can't output unescaped ) or |

This would be a commitment to only entering/exiting new contexts using whitespace or ASCII punctuators. That seems like it will not be a significant impediment to language evolution.