Compound Character Classes

Character classes are a versatile tool when combined with various pieces of the regex syntax. Compound character classes can help clarify and define sophisticated searches, test for certain conditions in a program, and filter wanted e-mail from spam. This section uses compound character classes to build meaningful expressions with the regex syntax.

Using compound character classes with the regex syntax.

Example 1: Find a partial e-mail address. Use a character class to denote a match for any number between 0 and 9. Use a range to restrict the number of times a digit matches.

Regex:
```
smith[0-9]{2}@
```
Matches:
```
smith44@
smith42@
```
Doesn't Match:
```
Smith34
smith6
Smith0a
```

Example 2: Search an HTML file to find each instance of a header tag. Allow matches on whitespace after the tag but before the ">".

Regex:
```
(<[Hh][1-6] *>)
```
Matches:
```
<H1>
<h6>
<H3  >
<h2    >
```
Doesn't Match:
```
<H1
<   h2>
<a1>
```

Example 3: Match a regular 7-digit phone number. Prevent the digit "0" from leading the string.

Regex:
```
([1-9][0-9]{2}-[0-9]{4})
```
Matches:
```
555-5555
123-4567
```
Doesn't Match:
```
555.5555
1234-567
023-1234
```

Example 4: Match a valid web-based protocol. Escape the two front slashes.

Regex:
```
[a-z]+:\/\/
```
Matches:
```
http://
ftp://
tcl://
https://
```
Doesn't Match:
```
http
http:
1a3://
```

Example 5: Match a valid e-mail address.

Regex:

[a-z0-9_-]+(\.[a-z0-9_-]+)*@[a-z0-9_-]+(\.[a-z0-9_-]+)+

Matches:

j_smith@foo.com
j.smith@bc.canada.ca
smith99@foo.co.uk
1234@mydomain.net

Doesn't Match:
```
@foo.com
.smith@foo.net
smith.@foo.org
www.myemail.com
```
Note
This regular expression will actually also match the smith@foo.net part of the .smith@foo.net example.