The use for back references are somewhat limited, especially since you only have nine of them, but on some rare occasion you might need it. Note some regular expression implementations can use multi-digit numbers as long as they don't start with a 0. As you see only the left or right top subexpression will match, not both, this is sometimes handy when you want to run a complex pattern in one subexpression and if it fails try another one. Last we have sequences which defines sequences of characters which can match, sometimes you don't want match a word directly but rather something that resembles one. The sequence is somewhat similar with alternation, the similarity is that only one of the items listed will match.
Sequences are also mixed with quantifiers and assertions to produce more elaborate searches.
For instance matches all whole words. I'll give a brief explanation on how to convert them to regular expressions . After reading this article you probably have seen the similarities with wildcards. Altogether very similar to regular expressions. This gives .