A regular expression (regex or regexp for short) is a special text string for describing a search pattern. You can think of regular expressions as wildcards on steroids. You are probably familiar with wildcard notations such as *.txt to find all text files in a file manager. The regex equivalent is .*\.txt$. [1]
To understand and learn more about regular expressions and implementations of these, recommended read.
Footnotes
| [1] | Regular-Expressions.info |