expression implementations have back-reference bugs that can cause
programs to return incorrect answers or even crash, and fixing these
bugs has often been low-priority---for example, as of 2019 the GNU C
-library bug database contained back-reference bugs 52, 10844, 11053,
-and 25322, with little sign of forthcoming fixes. Luckily,
+library bug database contained back-reference bugs
+@url{https://sourceware.org/bugzilla/show_bug.cgi?id=52,52},
+@url{https://sourceware.org/bugzilla/show_bug.cgi?id=10844,10844},
+@url{https://sourceware.org/bugzilla/show_bug.cgi?id=11053,11053},
+and @url{https://sourceware.org/bugzilla/show_bug.cgi?id=25322,25322},
+with little sign of forthcoming fixes. Luckily,
back-references are rarely useful and it should be little trouble to
avoid them in practical applications.