Wonky STRING_MATCHES_REGEXP behaviour - is there a bug reporting thread or am I stupid?

As far as I understand from the docs, STRING_MATCHES_REGEXP should match always the whole string; therefore

~%string%~ STRING_MATCHES_REGEXP ~%regexp%~ 

and

~%string%~ STRING_MATCHES_REGEXP ~^%regexp%$~

should always be equivalent, unless ~%regexp%~ ends with an odd number of '\' characters.

However, I have:

BEGIN ~gist~
INSTALL_BY_DEFAULT
	ACTION_IF ~~ STRING_MATCHES_REGEXP ~^ $~ THEN BEGIN
		PRINT ~facepalm~
	END
	
/** Space, tab and new line with an optional carriage return character before it if built on a windows system. */
OUTER_TEXT_SPRINT WHITESPACE_CHARS ~ 		
~
/** Regexp for a sequence of ASCII white space characters. */
OUTER_TEXT_SPRINT WHITESPACE_RX ~\([%WHITESPACE_CHARS%]*\)~
/** Regexp for a character which is not ASCII white space. */
OUTER_TEXT_SPRINT NOT_WHITESPACE_RX ~\([^%WHITESPACE_CHARS%]*\)~

/** Regexp matching any sequence of '\' terminated by a character other than '"' and '\', 
  * or an odd sequence of '\' terminated by '"'. */
OUTER_TEXT_SPRINT JSON_STRING_ATOM_RX ~\(\\*[^\"]\|\([^\]\\\(\\\\\)*"\)\)~
/** Any string surrounded by a pair of '"' characters, in which all '"' are escaped with a '\'.*/
OUTER_TEXT_SPRINT JSON_STRING_RX ~"\(\(\\\(\\\\\)*"\)?\(%JSON_STRING_ATOM_RX%*\)\(\(\\\\\)*\)\)"~ 
	

DEFINE_ACTION_FUNCTION is_string
	STR_VAR json = ~~
	RET res
	BEGIN
		OUTER_SET res = 1
		ACTION_IF ~%json%~ STRING_MATCHES_REGEXP ~^%WHITESPACE_RX%%JSON_STRING_RX%%WHITESPACE_RX%$~ THEN BEGIN
			OUTER_SET res = 0
		END
	END

DEFINE_ACTION_FUNCTION is_string2
	STR_VAR json = ~~
	RET res
	BEGIN
		OUTER_SET res = 1
		ACTION_IF ~%json%~ STRING_MATCHES_REGEXP ~%WHITESPACE_RX%%JSON_STRING_RX%%WHITESPACE_RX%~ THEN BEGIN
			OUTER_SET res = 0
		END
	END

LAF is_string STR_VAR json = ~"""~ RET res END 
PRINT ~%res%~
LAF is_string2 STR_VAR json = ~"""~ RET res END
PRINT ~%res%~

Even worse,

~~ STRING_MATCHES_REGEXP ~^ $~

Prints:
facepalm

0

1

What should I do, other than go through all my weidu code and add those '^' and '$' to make it work?