This use of parentheses to store a pattern in memory is called a capture group. Every time a capture group is used (i.e., parentheses surround a pattern), the resulting match is stored in a numbered register. If you match two different sets of register parentheses, \\2 means whatever matched the second capture group. Thus /the (.*)er they (.*), the \\1er we \\2/ will match the faster they ran, the faster we ran but not the faster they ran, the faster we ate. Similarly, the third capture group is stored in \\3, the fourth is \\4, and so on.