About Preg Match patterns

hi, i have a question, i dont know what keywords to search, so im asking here. while using preg_match, there are lots of thing that can be written as $pattern. for example: preg_match("/php/i", "PHP is the web scripting language of choice.") what is the meaning of /i ? or '@^(?:http://)?([^/]+)@i' there are lots of chars here, or this: /(?P<name>\w+): (?P<digit>\d+)/ /[^\\d\\sa-zA-Z$_]/

/^[a-zA-Z\p{Cyrillic}0-9\s\-]+$/u    (there is a extra "/u" )

#^http:\/\/(.*)\.(gif|png|jpg)$#i #<head>(.*?)<\/head>#si there is lots and lots of different usage of these. my question is, where can i find a very detailed information about these ? i want and need to know whats what about these. Thanks, tolga.