rb_reg_backref_number  668 include/ruby/intern.h int rb_reg_backref_number(VALUE match, VALUE backref);
rb_reg_backref_number 4009 string.c               int nth = rb_reg_backref_number(match, backref);
rb_reg_backref_number 4258 string.c           nth = rb_reg_backref_number(match, backref);