$re = '/^---((?:\R(?!---$).*)*)\R---$/m';
$str = '
---
key: value
foo: bar, zip, grump
/*
Anything can go in here, once I have this section pulled out, the yaml schema
can do the reset. All that\'s important to note is that this section must be
terminated explicitly with a subsequent /^---\\s*$/ in order to be deemed valid.
---
Anything else can follow here, more accidental frontmatter blobs can exist,
but it should not matter since the other requirement is that the regex engine
will cease processing beyond the termination of the first match.
---
key: value
foo: bar, zip, grump
/*
Anything can go in here, once I have this section pulled out, the yaml schema
can do the reset. All that\'s important to note is that this section must be
terminated explicitly with a subsequent /^---\\s*$/ in order to be deemed valid.
---
Anything else can follow here, more accidental frontmatter blobs can exist,
but it should not matter since the other requirement is that the regex engine
will cease processing beyond the termination of the first match.';
preg_match_all($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