Regex in PHP

1. 1 Character 1 number and string length

^\S*(?=\S{8,})(?=\S*[a-z])(?=\S*[A-Z])(?=\S*[\d])\S*$

• ^: anchored to beginning of string
• \S*: any set of characters
• (?=\S{8,}): of at least length 8
• (?=\S*[a-z]): containing at least one lowercase letter
• (?=\S*[A-Z]): and at least one uppercase letter
• (?=\S*[\d]): and at least one number
• $: anchored to the end of the string

To include special characters, just add (?=\S*[\W]), which is non-word characters

OR

(?=^.{8,}$)((?=.*\d)|(?=.*\W+))(?![.\n])(?=.*[A-Z])(?=.*[a-z]).*$

   * contain at least (1) upper case letter
   * contain at least (1) lower case letter
   * contain at least (1) number or special character
   * contain at least (8) characters in length

References
1. http://www.autohotkey.com/docs/misc/RegEx-QuickRef.htm
2. http://www.regular-expressions.info/index.html