This enormous regexp matches any "Geometry" parameter for ImageMagick. See the docs about this: http://www.imagemagick.org/script/command-line-processing.php#geometry

imagemagick_geometry.php

<?php

 $number = "\d*(?:\.\d+)?"; // It's a reference to use in other cases that matches any kind of number/float
 
 $width = "(?<w>(?:$number)?%?)?"; // This is the first part, the width
 $height = "(?:x(?<h>(?:$number)?%?))?"; // Here is the height, the same as "width" but starting with an "x"
 
 $aspect = "[!><@^]"; // These are the different filters one can use to stretch, shrink, etc.
 
 $size = "$width$height"; // To match any size we need width and height at least (aspect comes later)
 
 $offset = "(?<x>[+-]$number)?(?<y>[+-]$number)?"; // This is the geometry offset
 
 $regexp = "(?<size>$size)(?<aspect>$aspect)?(?<offset>$offset)"; // Here we have the full regexp
 
 echo $regexp;
 
 // Should echo this :
 // (?<size>(?<w>(?:\d*(?:\.\d+)?)?%?)?(?:x(?<h>(?:\d*(?:\.\d+)?)?%?))?)(?<aspect>[!><@^])?(?<offset>(?<x>[+-]\d*(?:\.\d+)?)?(?<y>[+-]\d*(?:\.\d+)?)?)