$re = '/# match a body pattern alone on a line
^\g<body>$
# we\'ll use (?<name>...){0} to define the pattern \g<name>
# without having to match the pattern at the current position
# a body pattern matches
(?<body>
# a sequence of
(?: [^\[\]{}()\n] # non-delimiters
| \g<parens> # parenthesized pattern
| \g<square_brackets> # bracketed pattern
| \g<curly_braces> # braced pattern
)*
){0}
# a parenthesized pattern matches
(?<parens>
\( # an open paren
\g<body> # a valid body
\) # a close paren
){0}
# a bracketed pattern matches
(?<square_brackets>
\[ # an open square bracket
\g<body> # a valid body
\] # a close square bracket
){0}
# a braced pattern matches
(?<curly_braces>
\{ # an open curly brace
\g<body> # a valid body
\} # a close curly brace
){0}/mx';
$str = 'matches:
a plain string
a string containing (matched parens) [matched brackets] and {matched braces}
nested (parens (within) parens)
nested [parens (within) brackets]
()()[({({[]})[({})]})]
non-matches:
(unbalanced parens
([balanced but misordered)]';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php