org-babel-src-block-regexp

org-babel-src-block-regexp is a variable defined in `ob-core.el'.
Its value is

"^\\([ 	]*\\)#\\+begin_src[ 	]+\\([^ \f	\n
]+\\)[ 	]*\\([^\":\n]*\"[^\"\n*]*\"[^\":\n]*\\|[^\":\n]*\\)\\([^\n]*\\)\n\\([^]*?\n\\)??[ 	]*#\\+end_src"

Documentation:
Regexp used to identify code blocks.