How is regex implemented in gawk?

[Peng Yu]

Hi,

regex.h supports REG_EXTENDED.

https://www.gnu.org/software/libc/manual/html_node/Flags-for-POSIX-Regexps.html

In the following file, it says "In addition to the POSIX-Extended
features the escape character is special inside a character class
declaration.

In addition, some escape sequences that are not defined as part of
POSIX-Extended specification are required to be supported."

https://www.boost.org/doc/libs/1_34_1/libs/regex/doc/syntax_extended.html

Are these the following differences between awk's regex and REG_EXTENDED?

Since the difference between awk's regex and REG_EXTENDED is not much?
How is awk's regex implemented? Is there a way to reuse some
implementation in REG_EXTENDED and only add some minimal code include
these extra features? Or the implementation regex in gawk starts from
scratch? Thanks.

-- 
Regards,
Peng