org-match-substring-with-braces-regexp

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

"\\(\\S-\\)\\([_^]\\)\\(\\({\\([^{}]*?\\|\\(?:[^{}]*?{[^{}]*?}\\)+[^{}]*?\\|\\(?:[^{}]*?{\\(?:[^{}]*?{[^{}]*?}\\)+[^{}]*?}\\)+[^{}]*?\\)}\\)\\)"

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