Consider the string,
"This is a man".
Does the sting have the word "man"?
If it the string has the word "man," can you change it to "woman"?
The Word RegExp
In the above example, "man" is a RegExp. More generally, a RegExp is a string (usually a small string) of characters for which you want to know whether it exists in some available string. This available string might have been assigned to a variable.
When the RegExp is seen in the available string, we say matching has occurred. That is, the RegExp has matched the string. When matching occurs, replacement can follow. If the regExp "man" in the above example is seen, it can be replaced by the word "woman."
Modern and Old-Fashioned Ways of Coding RegExps
At first, to answer the above types of questions, you had to do the coding using programming basics (declaration of variable, conditions, loops, etc). These questions can be classified (grouped). The computer language Perl came up with functions to handle the above questions, which gives the programmer less work.