I think this regex will work :
@class:\s*rtm_due\nspan:\s*\[{.*}, {'#text':\s*(.*),\s*'@class':\s*'rtm_due_value'}]
Demo herebut for only due date
If you want location
you need to chagne regex to :
@class:\s*rtm_location\nspan:\s*\[{.*}, {'#text':(.*),\s*'@class':\s*'rtm_location_value'}]
the group 1 should give the desired value.
This is the output I got in one of the php regex testers available online :
[0] => @class: rtm_duespan: [{'#text': 'Due:', '@class': 'rtm_due_title'}, {'#text': 'Sat 16 Jul 16', '@class': 'rtm_due_value'}] [1] => 'Sat 16 Jul 16'