I'm replacing the current regular expression library with modern library(pcre) for mariadb. I need some information about the ways that code uses the old regex library. The relevant codes are in sql/item_cmpfunc.cc directory. For exapmle in the function : int Item_func_regex::regcomp(bool send_error) I can't understand most of the lines, and the code is not well-commented. In the new library(pcre) uses different way to compile a regEx so I have to understand this and code for pcre preserving the functionalists of the class. Is there any documentation for this file or the way they used to work with old regex library ? thanks.