$re = '/(?<function>\w+) \( # Matches function name
(?<parameters> # Simple param type.
(?<data> # Simple data type: inner function or word (keep this order!).
(?R) | # Or recursive this Regexp to match inner functions.
\w+ # Matches a simple word.
)
( # Match additional parameters:
\s*,\s* # Match comma, but skip spaces.
\g<parameters> # Recurse additional parameters.
)* # Zero or more additional parameters.
)? # Parameters are optional
\)/mx';
$str = 'color()
color(a)
color(a, a)
color(a())
color(a(b()))
color(a, a())
color(a, a(), b)
color(a(), b)
color(a, b, c(d, e(f, g()), h))';
preg_match_all($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