Understanding Regular Expression

[priti]

Hi All,

 

I have a piece of code from php.net

 

if (!ctype_alnum($username) || !preg_match('/^(?:[a-z0-9_-]|\.(?!\.))+$/iD', $userfile)) {
   die("Bad username/filename");
}

 

I do not understand "/D" ! I know of /g, /i OR /m for multi line..

 

can someone please help me understanding this expression.

Edited October 8, 2012 by priti

[requinix]

Pattern Modifiers

[priti]

Pattern Modifiers

 

Thanks for /D .Can you please also help me understanding the regular expression

 

 

preg_match('/^(?:[a-z0-9_-]|\.(?!\.))+$/iD

[requinix]

Flags aside,

"

^
 beginning of the string (sometimes line)
(?:...)
 group but don't capture
[a-z0-9_-]
 letters (including uppercase since you have /i), numbers, underscores, and hyphens
|
 or ('x|y' means 'x or y')
\.
 literal period
(?!...)
 negative lookahead ('the following must not be ...')
+
 one or more of the previous, which in this case applies to the first group

Summed up, the expression matches any sequence of letters/numbers/underscores/hyphens/periods but won't allow two (or more) consecutive periods.

Edited October 8, 2012 by requinix

[Christian F.]

Also, using both ctype_alnum () and preg_match () is not necessary at all. The Regular Expression alone is enough.

In fact, the Regular Expression will never have any meaning in the code you posted, as the first command is way more restrictive. Since a period is not a part of the "alpha numerical" definition, the RegExp will never have the chance to test for periods.