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:

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

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.

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 [-+_~.\d\w]:

• - 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 =.

Answer 2

I've decided to go you one better and explain the entire regex:

^                 # anchor to start of line (                 # start grouping  (                # start grouping   [\w]+           # at least one of 0-9a-zA-Z_   :               # a literal colon  )                # end grouping  ?                # this grouping is optional  \/\/             # two literal slashes )                 # end capture ?                 # this grouping is optional (  (   [\d\w]          # exactly one of 0-9a-zA-Z_                   # having \d is redundant   |               # alternation   %               # literal % sign   [a-fA-f\d]{2,2} # exactly 2 hexadecimal digits                   # should probably be A-F                   # using {2} would have sufficed  )+               # at least one of these groups  (                # start grouping   :               # literal colon   (    [\d\w]    |    %    [a-fA-f\d]{2,2}   )+  )?               # Same grouping, but it is optional                   # and there can be only one  @                # literal @ sign )?                # this group is optional (  [\d\w]           # same as [\w], explained above  [-\d\w]{0,253}   # includes a dash (-) as a valid character                   # between 0 and 253 of these characters  [\d\w]           # end with \w.  They want at most 255                   # total and - cannot be at the start                   # or end  \.               # literal period )+                # at least one of these groups [\w]{2,4}         # two to four \w characters (  :                # literal colon  [\d]+            # at least one digit )? (  \/               # literal slash  (   [-+_~.\d\w]    # one of these characters   |              # *or*   %              # % with two hex digit combo   [a-fA-f\d]{2,2}  )*              # zero or more of these groups )*               # zero or more of these groups (  \?              # literal question mark  (   &?         # literal &amp or & (semicolon optional)   (    [-+_~.\d\w]    |    %    [a-fA-f\d]{2,2}   )   =?             # optional literal =  )*              # zero or more of this group )?               # this group is optional (  #               # literal #  (   [-+_~.\d\w]   |   %   [a-fA-f\d]{2,2}  )* )? $                # anchor to end of line 

It's important to understand what the metacharacters/sequences are. Some sequences are not meta when used in certain contexts (especially a character class). I've cataloged them for you:

meta with no context

• ^ -- zero width start of line
• () -- grouping/capture
• ? -- zero or one of the preceding sequence
• + -- one or more of the preceding sequence
• * -- zero or more of the preceding sequence
• [] -- character class
• \w -- alphanumeric characters and _. Opposite of \W
• | -- alternation
• {} -- length assertion
• $ -- zero width end of line

This excludes :, @, and % from having any special/meta meaning in the raw context.

meta inside character class

] ends the character class. - creates a range of characters unless it is at the start or the end of the character class or escaped with a backslash.

grouping assertions

A (? combination starts a grouping assertion. For example, (?: means group but do not capture. This means that in the regex /(?:a)/, it will match the string "a", but a is not captured for use in replacement or match groups as it would be from /(a)/.

? can also be used for lookahead/lookbehind assertions with ?=, ?!, ?<=, ?<!. (? followed by any sequence except what I mentioned in this section is just a literal ?.