use of colon symbol in regular expression - regex

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 ?.

Answer 3

There is no special use for colon : in your case :
(([\w]+:)?\/\/)? will match http://, https://, ftp://...
You can find one special use for colon : every capturing group starting by (?: won't appear in the results.
Example, with "foobarbaz" in input :
/foo((bar)(baz))/ => { [1] => 'barbaz', [2] => 'bar', [3] => 'baz' }
/foo(?:(bar)(baz))/ => { [1] => 'bar', [2] => 'baz' }

Answer 4

A colon has no special meaning in Regular Expressions, it just matches a literal colon.
[\w]+:
This just means any word character 1 or more times followed by a literal colon
The brackets are actually not needed here. Square brackets are used to define a group of characters to match. So
[abcd]
means a single character of a, b, c, d