Generate date range regex, PHP.

date_range_regex.php

function get_daterange_regex( $from, $to, $input_format ) {

	// get the difference in months
	$date1 = DateTime::createFromFormat( $from, $input_format );
	$date2 = DateTime::createFromFormat( $to, $input_format );
	$interval = $date1->diff($date2);
	$months = $interval->m;
	
	// get an array of the interveening months
	$all_months = array();
	for ($i = 0; $i<=$months; $i ++) {
		$all_months[$i] = $date1->modify('+1 month');
	}
	$month_codes = array_map( 'acf_date_format', $all_months);
		
	// return the regex string created from the array
	return "/" . implode( "|", $month_codes ) . "/";
}
function acf_date_format($date) {
	return date_format($date, 'Ym');
}

// e.g Get the regex string for finding posts between Dec 2011 and Mar 2012
$date_regex = get_daterange_regex( '2011-12', '2012-03', 'Y-m' );

// Outputs "/201112|201201|201202|201203/" depending on date formats given
echo $date_regex