When creating regular expressions for PHP I tend to use the awesome tool PHP Live Regex.
This tool has really helped me over the last couple of years to create regular expressions for many of the projects I've been working on. Everything from validation functions to formatters that use regular expressions have had their expressions worked out using this tool. I even use it for testing one off expressions where I just need to find/replace in a file.
Encoding special characters in a block of HTML or other code can be a pain because there might already be ampersands there that impart encoding. This might be an ampersand that has already been encoded with a &, or it might be an ampersand in the code as an if statement or similar.
Use the following regular expression to find any ampersand that hasn't already been encoded.
([^&])&(?!#?[a-zA-Z0-9]{2,6};|\$|&)When using replace, you can turn any ampersand into & by using the following replace.
$1&The only problem with this statement is when the code uses a & operator as part of a statement to do bitwise operations.
Here are some of the regular expressions that I frequently use.
Find a blank line
^$Spaces
[ \t]+You can use this to break a text string apart into words.
Date
\d{1,2}(\-|\/|\.)\d{1,2}\1\d{4}This will match anything in the format mm/dd/yyyy, or even dd/mm/yyyy.
[A-Z][a-z][a-z] [0-9][0-9]*, [0-9]{4}Will match a formatted date, such as Mar 24, 2007.
Time
([1-9]|1[0-2]):[0-5]\d(:[0-5]\d(\.\d{1,3})?)?This will match HH:MM or HH:MM:SS or HH:MM:SS.mmm.