用于过滤扩展的格式正确的正则表达式是什么样的？

How to match 4 extensions with regexp?

I have tired this:

$returnValue = preg_match('/(pdf|jpg|jpeg|tif)/', 'pdf', $matches);

I dont know why I get 2 matches? I'm I something missed in the regexp?

array (
  0 => 'pdf',
  1 => 'pdf',
)

I dont know why I get 2 matches

No you are getting only 1 match.

$matches has 2 entries:

1. 1st entry with index=0 is for the entire match of input by your regex
2. 2nd entry with index=1 is for the first matched group since your regex is enclosed in parentheses

If you want to avoid 2 entries you can use non-capturing group:

$returnValue = preg_match('/(?:pdf|jpg|jpeg|tif)/', 'pdf', $matches);

OR simply don't group them:

$returnValue = preg_match('/pdf|jpg|jpeg|tif/', 'pdf', $matches);