$和*在PCRE（正则表达式）中意味着什么？

Question

i am using a Regular Expression that validates an email address here is the regular expression i am using.

preg_match("/^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$/", $email)

most of the above code are self explanatory like

a) ^ represents NOT.

b) the start of the string should be either _ a-z 0-9

c) match the next character which starts with dot

d) now what does *@ means here, couldn't it be just @ which means the next character should be @

e) next again it will try and find dot, the first dot is optional and the second is compulsory.

f) in the end what does $ means?

Answer 1

Your assumption a) is not true

^ is the start of the string in this case. At the beginning of a character class its a NOT.

[_a-z0-9-]+ will match any of the chars in [] one or more times (because of the +)

(\.[_a-z0-9-]+)* then there is a dot the same pattern than before and the * means this complete part can be repeated 0 or more times

Then there has to be the character @

Then the part from before the @ repeats

(\.[a-z]{2,3})$ the string has to end (defined by the $) with a . and 2 or 3 lowercase letters