In the book of D. Koterov there is a chapter "pockets of regulars", but little understand what it is. Maybe a mini lecture someone will hold or links, or books, where it is described.
Maybe it has a different name?
Almost everything between // enclosed in parentheses will be stored in special temporary variables. The strange name "pockets" I hear for the first time, but oh well. /..../ corresponds to zero pocket. That is, everything that coincides with the full expression falls into the $ 0 variable. ([abc] +) is the first pocket. ([0-9] +) - the second. (?: [a-z0-9] +) - none at all, since the word?: says that this grouping should not be remembered.
To understand this in more detail, make a complex regular schedule, make a
preg_match_all for a long text, and then print the result using