The square brackets are required. The following string represents one word in the typical username. Basically, a word is either an atom or quoted string. Lost data is by far the most expensive, with backup problems which must be diagnosed and fixed coming a fairly close second. One simple precaution to help prevent both problems is to replace your backup tapes at least as often as the manufacturer recommends.
Modern tapes are high-precision devices that work in very stressful environments.
For example Ultrabac recommends replacing 4 and 8mm tapes after no more than 50 uses. Exabyte says its Mammoth 2 8mm tape is good for 1500 passes. Quantum measures its DLT tape's durability in passes but provides a method of converting that into uses. The easiest way to keep a replacement schedule is by calendar date rather than hours of use or numbers of backups. With a little calculation you can convert hours or backup numbers into a date when your tape will need replacement.