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Some commonly used regex examples.

First, to match something that isn't the regex, aka What is the regular expression to not match a pattern, or how do I regex for something that doesn't match:

^(?!regexhere).*$

The example above would match on a string that does not equal "regexhere".

Below is an example for an IP address:

^(?!192\.168\.1\.1).*$

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Below is a quick example to do simple IP validation:

Long way: ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$
Short way: ^([0-9]{1,3}(.[^$]|$)){4}

That's just: ^ (starts with) [0-9]{1,3} the numbers 0-9 in length of 1 to 3, 4 times delimited by decimals, ending in a decimal or the end of the string, but not ending in a decimal at the end of the string.

 

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Lookahead / Lookbehind / Lookaround Example

(?<!href)="http://[^\s]+"

The expression above will match anything pointing to an http:// url except links. It does this by matching a url attribute ="http://[^\s]+" that is not preceded by href (ie: src=, etc) . References to urls that are not attributes should be ignored. This is useful if you want to search for images that are not https, etc.