# Regular expressions accepted by programming languages but are not regular [duplicate]

The below definition of Regular Language is given in Wikipedia.

In theoretical computer science and formal language theory, a regular language (also called a rational language) is a formal language that can be expressed using a regular expression, in the strict sense of the latter notion used in theoretical computer science (as opposed to many regular expressions engines provided by modern programming languages, which are augmented with features that allow recognition of languages that cannot be expressed by a classic regular expression).

The sentence in bold says that there are languages which are not regular but still are recognized by regular expressions engines of modern programming languages. Can anybody provide me an example of such a language?

• The quoted paragraph includes a link to a section on the article headed "Patterns for non-regular languages" which gives an example: (.+)\1 using PCRE syntax. Sep 16 '20 at 2:57