When should you use regex-based step definitions versus Cucumber Expressions?

Cucumber Expressions cover the vast majority of everyday step-matching needs, string, number, and word parameters, with substantially more readable syntax than an equivalent regular expression, and should be the default choice for new step definitions.

Regular expressions remain the better (or only) choice when a step needs pattern matching Cucumber Expressions don't directly support: alternation within a single word (matching either "logs in" or "signs in" with one pattern), optional text segments, or highly specific character-class constraints that would require defining a bespoke custom parameter type to express as a Cucumber Expression anyway.

// Cucumber Expression handles the common case cleanly
@Given("a user named {string}")

// Regex needed for alternation within a single step
@Given("^the user (logs in|signs in) successfully$")

A reasonable default is: reach for Cucumber Expressions first, and drop down to regex specifically for the step definitions where its pattern-matching power is genuinely needed, rather than defaulting to regex everywhere out of habit and losing the readability benefit Cucumber Expressions were designed to provide.