^        # Assert position at the beginning of the string.
\(       # Match a literal "("
  ?      #   between zero and one time.
(        # Capture the enclosed match to backreference 1:
  [0-9]  #   Match a digit
    {3}  #     exactly three times.
)        # End capturing group 1.
\)       # Match a literal ")"
  ?      #   between zero and one time.
[-. ]    # Match one hyphen, dot, or space
  ?      #   between zero and one time.
⋯       # [Match the remaining digits and separator.]
$        # Assert position at the end of the string.

Let’s look at each of these parts more closely.

The ‹^› and ‹$› at the beginning and end of the regular expression are a special kind of metacharacter called an anchor or assertion. Instead of matching text, assertions match a position within the text. Specifically, ‹^› matches at the beginning of the text, and ‹$› at the end. This ensures that the phone number regex does not match within longer text, such as 123-456-78901.

As we’ve repeatedly seen, parentheses are special characters in regular expressions, but in this case we want to allow a user to enter parentheses and have our regex recognize them. This is a textbook example of where we need a backslash to escape a special character so the regular expression treats it as literal input. Thus, the ‹\(› and ‹\)› sequences that enclose the first group of digits match literal parenthesis characters. Both are followed by a question mark, which makes them optional. We’ll explain more about the question mark after discussing the other types of tokens in this regular expression.

The parentheses that appear without backslashes are capturing groups and are used to remember the values matched within them so that the matched text can be recalled later. In this case, backreferences to the captured values are used in the replacement text so we can easily reformat the phone number as needed.

Two other types of tokens used in this regular expression are character classes and quantifiers. Character classes allow you to match any one out of a set of characters. ‹[0-9]› is a character class that matches any digit. The regular expression flavors covered by this book all include the shorthand character class ‹\d› that also matches a digit, but in some flavors ‹\d› matches a digit from any language’s character set or script, which is not what we want here. See Recipe 2.3 for more information about ‹\d›.

‹[-.●]› is another character class, one that allows any one of three separators. It’s important that the hyphen appears first or last in this character class, because if it appeared between other characters, it would create a range, as with ‹[0-9]›. Another way to ensure that a hyphen inside a character class matches a literal version of itself is to escape it with a backslash. ‹[.\-●]› is therefore equivalent. The ‹●› represents a literal space character.

Finally, quantifiers allow you to repeatedly match a token or group. ‹{3}› is a quantifier that causes its preceding element to be matched exactly three times. The regular expression ‹[0-9]{3}› is therefore equivalent to ‹[0-9][0-9][0-9]›, but is shorter and hopefully easier to read. A question mark (mentioned earlier) is a quantifier that causes its preceding element to match zero or one time. It could also be written as ‹{0,1}›. Any quantifier that allows something to match zero times effectively makes that element optional. Since a question mark is used after each separator, the phone number digits are allowed to run together.