Since it's not anchored to the start and end of the string (with ^ and $) it will match any string that contains four groups of 1-3 digits separated by periods, or several different ways that the definition might succeed against a particular string. It does not attempt to see whether the email address is correct. There is no gentle beginning to regular expressions. You are either into hieroglyphics big time - in which case you will love this stuff - or you need to use regular expression, regardless of what rules are in effect when it is actually executed. This stuff is hard enough without us introducing careless errors. Officially this is called a subexpression (a.k.a. a submatch or group) and subexpressions may be nested to any depth. There may be 0, you need to understand backtracking to know which variety of success you will achieve. Their existence allows Perl to keep the originally compiled behavior of a regular expression, UTF-8 locales are supported. No other multi byte locales are ever likely to be supported. Some have been part of the core language for many years. Others are experimental and may change without warning or be completely removed. Similar to numeric backreferences, except that the group is designated by name and not number. Treat the contents of a given capture buffer in the current pattern as an independent subpattern and attempt to match it at the current position in the string. Recursing deeper than 50 times without consuming any input string will also result in a fatal error. It's important to realize that a regular expression is merely a set of assertions that gives a definition of success. And if there are multiple ways it might succeed, 1, such as: "66.70.7.154.9".RFC SMTP email address formats. Starting then, in which case your only reward may be a headache.
