syntax: full support for a regexp following =~

[mvdan]

Right now in extended tests we simply read a word after =~. This works most of the time, but it can be broken with some forms of ERE, by using characters that would otherwise be tokens (like () or spaces.

[[ foo =~ bar(a) ]]
[[ foo =~ ba[ r] ]]

[mvdan]

Forgot to link to this issue in 4ea5a56. Still not there, but support is better.

[mvdan]

What we're doing now (after the commit above) is read until we find ]]. This will work most of the time, but will break if a tab is used instead of a space, or if the regex itself contains ]].

 $ echo '[[ a =~ ( ]]) ]]' | bash
 $ echo '[[ a =~ ( ]]) ]]' | shfmt
1:13: statements must be separated by &, ; or a newline