Can someone explain this regular expression?

/^[\p{Ll}\p{Lm}\p{Lo}\p{Lt}\p{Lu}\p{Nd}]+$/mu

This is the regular expression validation that cakePHP uses to validate a开发者_JAVA技巧lphanumeric strings. I am unable to understand what Ll, Lm, Lt etc are? This is to validate alphanumeric strings, so they should test for numbers and characters. Could someone explain this expression a little.

Thank you.

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Ll, Lm, Lo, Lt, Lu, Nd are unicode character classes.

See here at around 1/3 of the page:

http://www.regular-expressions.info/unicode.html

• \p{Ll} or \p{Lowercase_Letter}: a lowercase letter that has an uppercase variant.
• \p{Lu} or \p{Uppercase_Letter}: an uppercase letter that has a lowercase variant.
• \p{Lt} or \p{Titlecase_Letter}: a letter that appears at the start of a word when only the first letter of the word is capitalized.
• \p{L&} or \p{Letter&}: a letter that exists in lowercase and uppercase variants (combination of Ll, Lu and Lt).
• \p{Lm} or \p{Modifier_Letter}: a special character that is used like a letter.
• \p{Lo} or \p{Other_Letter}: a letter or ideograph that does not have lowercase and uppercase variants.

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The code between the curly brackets (Li, Lm, Lt, etc) are classes of Unicode characters. A quick google for Unicode character classes produces for example the following list: http://www.siao2.com/2005/04/23/411106.aspx

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If you regularily stumble upon weird regular expressions, try one of these: https://stackoverflow.com/questions/89718/is-there-anything-like-regexbuddy-in-the-open-source-world - albeit I'm not sure if they explain those (mostly Unicode?) placeholders. Otherwise check out the list on http://regular-expressions.info/