Background Info on Regular Expressions
This is what Wikipedia has to say about them:
In computing, regular expressions provide a concise and flexible means for identifying strings of text of interest, such as particular characters, words, or patterns of characters. Regular expressions (abbreviated as regex or regexp, with plural forms regexes, regexps, or regexen) are written in a formal language that can be interpreted by a regular expression processor, a program that either serves as a parser generator or examines text and identifies parts that match the provided specification.
Now, that doesn’t really tell me much about the actual patterns. The regexes I’ll be going over today contains characters such as \w, \s, \1, and many others that represent something totally different from what they look like.
If you’d like to learn a little about regular expressions before you continue reading this article, I’d suggest watching the Regular Expressions for Dummies screencast series.
The eight regular expressions we’ll be going over today will allow you to match a(n): username, password, email, hex value (like #fff or #000), slug, URL, IP address, and an HTML tag. As the list goes down, the regular expressions get more and more confusing. The pictures for each regex in the beginning are easy to follow, but the last four are more easily understood by reading the explanation.
The key thing to remember about regular expressions is that they are almost read forwards and backwards at the same time. This sentence will make more sense when we talk about matching HTML tags.
Note: The delimiters used in the regular expressions are forward slashes, "/". Each pattern begins and ends with a delimiter. If a forward slash appears in a regex, we must escape it with a backslash: "\/".
Matching a Username

Pattern:
- /^[a-z0-9_-]{3,16}$/
/^[a-z0-9_-]{3,16}$/
Read more…