The term regular expressions refers to a type of search language. You'd use it to search for text strings that match certain defined patterns. So, if you need to search through a document for email addresses, IP addresses, credit card numbers, Social Security numbers, etc., you'd define a regular expression. For example, let's say that you need to search through system log files for all lines that contain a valid IP address. You'd create a regular expression that looks something like:
\b(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\b
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