regex - use of colon symbol in regular expression

Question

I am new to regex. I am studying it in regularexperssion.com. The question is that I need to know what is the use of a colon (:) in regular expressions.

For example:

$pattern = '/^(([w]+:)?//)?(([dw]|%[a-fA-fd]{2,2})+(:([dw]|%[a-fA-fd]{2,2})+)?@)?([dw][-dw]{0,253}[dw].)+[w]{2,4}(:[d]+)?(/([-+_~.dw]|%[a-fA-fd]{2,2})*)*(?(&?([-+_~.dw]|%[a-fA-fd]{2,2})=?)*)?(#([-+_~.dw]|%[a-fA-fd]{2,2})*)?$/';

which matches:

$url1  = "http://www.somewebsite.com";
$url2  = "https://www.somewebsite.com";
$url3  = "https://somewebsite.com";
$url4  = "www.somewebsite.com";
$url5  = "somewebsite.com";

Yeah, any help would be greatly appreciated.

question from:https://stackoverflow.com/questions/21045297/use-of-colon-symbol-in-regular-expression

Answer 1

Colon : is simply colon. It means nothing, except special cases like, for example, clustering without capturing (also known as a non-capturing group):

(?:pattern)

Also it can be used in character classes, for example:

[[:upper:]]

However, in your case colon is just a colon.

Special characters used in your regex:

In character class [-+_~.dw]:

• - means -
• + means +
• _ means _
• ~ means ~
• . means .
• d means any digit
• w means any word character

These symbols have this meaning because they are used in a symbol class []. Without symbol class + and . have special meaning.

Other elements:

• =? means = that can occur 0 or 1 times; in other words = that can occur or not, optional =.