Download - The Bastards Book of Regular Expressions

Changing phone format (TODO) 139AnswerFind (\d{3})[ \-]?(\d{3})[ \-]?(\d{4})Replace :(\1)-\2-\3Let’s focus on the regex we use to match either a hyphen or a whitespace character:[ \-]So there’s a space in there. But what’s with the backslash-hyphen? Recall that hyphens, when insidesquare brackets, become a special character. For example, [a-z], is the character set of lowercaseletters from a to z.Thus, when we want to include a literal hyphen inside a square-bracketed character set, we need toescape it with a backslash.One more thing to note: the square brackets act as a sort of grouping: thus, the ? makes optionaleither the hyphen or whitespace character (and whatever other characters we want to include inthose square brackets).Exercise: Exactly 10 digits So our web-form will now correctly handle a 10-digit phone numberthat may or may not have hyphens/spaces.But what happens when the user enters in more than 10-digits? If you apply the previous solutionto the following number:25556164040The pattern will match this and the replacement will end up as:(255)-561-64040In a real-life application, we do not want our regex to be flexible enough to accept this. Why?Because this kind of input indicates that there is an error by the user. Maybe that initial 2 is a typo.Or maybe his finger slipped at the end and added an extra 0. Either way, we do not want our regexto match this. We want the user to know that he possibly screwed up.