Properly quote file names in the regexp used to extract backup times.

If a backup filename contains certain special characters, then an
exception is thrown when used in the regular expression to extract the
time from the backup filename. To protect against this the metaquoting
escape sequence (`\Q...\E`) is needed.

This fixes issue #2680.

Author: drgrice1

lib/WeBWorK/ContentGenerator/Instructor/PGProblemEditor.pm

@@ -620,7 +620,7 @@ sub getBackupTimes ($c) {
 	my $backupBasePath = $c->{backupBasePath};
 	my @files          = glob(qq("$backupBasePath*"));
 	return unless @files;
-	return reverse(map { $_ =~ s/$backupBasePath//r } @files);
+	return reverse(map { $_ =~ s/\Q$backupBasePath\E//r } @files);
 }
 
 sub backupFile ($c, $outputFilePath) {