Definitions for "Regular expression"
A formal syntactic specification widely implemented in the Unix language family for reference to strings. For example, the regular expression denoting a string of alphanumerics (i.e., letters or numbers) is [A-Za-z0-9
An expression defined using operators that specifies what characters to expect in an input unit and in which order. These are used to define certain lexical units that you wish to recognize, such as dates, prices, etc.
(RE) A string of characters specifying a pattern. Usually, wildcards (match to anything) and variable positions (match to one of the following) are allowed.