Fortunately, there are libraries of well defined regular expressions from which to start with, so it isn't so intimidating.
http://www.expertsrt.net/m
Is an excellent place to start.
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PHP, check string for invalid character?
How can I check if a string contains symbols or weird characters like !@#$%^%^&&*(())-+=?><,.'"}
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by: HackneyCabPosted on 2007-08-12 at 04:16:51ID: 19678784
A better way to do things is to check that a string only contains the characters that are allowed. This is known as using a whitelist. Checking for illegal characters is known as using a blacklist. A whitelist is much easier to maintain than a blacklist.
Using regular expressions is a quick and easy way to check for character classes, so take a look at preg_match in the PHP documentation.
For instance, if your string only permitted letters, digits and standard spaces, you could use this to check that the string was valid:
$valid = preg_match('/^[a-z0-9 ]+$/', $string);
This does not allow an empty string, but if you change the + to a * the empty string will be permitted. If the string is valid, then $valid will equal true.
Regular expressions are not easy to grasp at first, but they are very powerful for string validation and manipulation.