A regular expression is a sequence of characters that specifies a match pattern in text. Among the most important concepts are:
* Character classes: defines a set of characters, any one of which can occur in an input string for a match to succeed.
* Quantifiers: used to specify how many instances of a character, group, or character class must be present in the input for a match.
* Wildcard (`.`): matches all characters except line terminators (also matches them if the `s` flag is set).
Many of these features include shortcuts of widely used expressions, so there is more than one way to construct a regular expression to achieve the same results.
For example, to match a two-digit number, one could write `+[0-9]{2,2}+` or `+\d{2}+`. The latter is not only shorter but easier to read and thus to maintain.
This rule recommends replacing some quantifiers and character classes with more concise equivalents:
