If you string several characters in a row, they must match in order. As we proceed, we will discuss much more reliable patterns for e-mail verification. Some characters don't match themselves, but are metacharacters . You can match these characters literally by placing a backslash in front of them. A backslash also turns an alphanumeric character into a metacharacter.
Some sequences are actually zero characters wide.
Regular expression are mostly assertions, i.e. plain characters that simply assert that they match themselves. As a matter of fact, most of our explanations are based on this great book. Any of those strings can trigger a match. Quantifiers say how many of the previous substring should match in a row.