Why is this an issue?

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:

• \d for [0-9] and \D for [^0-9]
• \w for [A-Za-z0-9_] and \W for [^A-Za-z0-9_]
• . for character classes matching everything (e.g. [\w\W], [\d\D], or [\s\S] with s flag)
• x? for x{0,1}, x* for x{0,}, x+ for x{1,}, x{N} for x{N,N}

r"[0-9]"        # Noncompliant - same as r"\d"
r"[^0-9]"       # Noncompliant - same as r"\D"
r"[A-Za-z0-9_]" # Noncompliant - same as r"\w"
r"[\w\W]"       # Noncompliant - same as r"."
r"a{0,}"        # Noncompliant - same as r"a*"

Use the more concise version to make the regex expression more readable.

r"\d"
r"\D"
r"\w"
r"."
r"a*"