A regular expression is a set of pattern matching rules encoded in a string according to certain syntax rules. Although the syntax is somewhat complex it is very powerful and allows much more useful pattern matching than say simple wild cards like ? and *.
Regular expressions are useful in the production of syntax highlighting systems, data validation, and many other tasks.
The below link explains the 8 regular expressions you should know for your projects. Here the expressions are explained by using the graphical pictures hence easily can understand.
http://net.tutsplus.com/tutorials/other/8-regular-expressions-you-should-know/
You can also get the Regular Expression Cheat Sheet for reference.
Ganesh's Blog 