Recursive RegEx (?R), (?-2)

• ?> は Atomic Grouping
• (?-2) は Recursion
• Atomic Grouping と Recursion は常にペアで使う。
• (?-2) は "Call recursively 2 outside parenthesis"
• (?-2) が示す parenthesis を non-capture (?:...)にすることはできない。

/(@[^@{};]+(?:;|(({(?:(?>[^{}"']+|\s+|"(?:\\"|\\\n|[^"\n])*?"|'(?:\\'|\\\n|[^'\n])*?')|(?-2))*})))|[^@{}\s][^@{}]*(({(?:(?>[^{}"']+|\s+|"(?:\\"|\\\n|[^"\n])*?"|'(?:\\'|\\\n|[^'\n])*?')|(?-2))*})))/su

@charset "utf-8";

@IDENTIFIER (RULE);

@supports (display: flex) {
  @media screen and (min-width: 900px) {
    article {
      display: flex;
    }
  }
}

.bad-block {{}

.simplest-block{}

#main .header {
	width:100px;
}

.nested-block {{}}

.complex>.selector {
	abc { "this is a string" }
}

.good-multi-line-string {
	abc {
		'1234\
			5678'
	} xyz
}

.bad-multi-line-string {
	abc {
		'1234
			5678'
	} xyz
}

.string-with-escape { abc { '12}34\'5678' } xyz } remaining

Array
(
    [0] => @charset "utf-8";
    [1] => @IDENTIFIER (RULE);
    [2] => @supports (display: flex) {
  @media screen and (min-width: 900px) {
    article {
      display: flex;
    }
  }
}
    [3] => .simplest-block{}
    [4] => #main .header {
	width:100px;
}
    [5] => .nested-block {{}}
    [6] => .complex>.selector {
	abc { "this is a string" }
}
    [7] => .good-multi-line-string {
	abc {
		'1234\
			5678'
	} xyz
}
    [8] => .string-with-escape { abc { '12}34\'5678' } xyz }
)