org-match-substring-regexp

org-match-substring-regexp is a variable defined in `org.el'.
Its value is

"\\(\\S-\\)\\([_^]\\)\\(\\({\\([^{}]*?\\|\\(?:[^{}]*?{[^{}]*?}\\)+[^{}]*?\\|\\(?:[^{}]*?{\\(?:[^{}]*?{[^{}]*?}\\)+[^{}]*?}\\)+[^{}]*?\\)}\\)\\|\\((\\([^()]*?\\|\\(?:[^()]*?([^()]*?)\\)+[^()]*?\\|\\(?:[^()]*?(\\(?:[^()]*?([^()]*?)\\)+[^()]*?)\\)+[^()]*?\\))\\)\\|\\(\\(?:\\*\\|[-+]?[^-+*!@#$%^_ 	
\n,:\"?<>~;./{}=()]+\\)\\)\\)"

Documentation:
The regular expression matching a sub- or superscript.