Why does this not match my example? - regex

Question

as I go through the regex101 quiz/lessons, I am supposed to match an IP address (without leading zeros).
Now the following
^[^0]+[0-9]+\\.[^0]+[0-9]+\\.[^0]+[0-9]+\\.[^0]+[0-9]+$
matches 23.34.7433.33
but fails to match single digit numbers like 1.2.3.4
Why is this so, when my + is supposed to match "1 to infinite" times...?

Answer 1

You are in fact matching more than 2 digits for each number in the IP address because you have:
[^0]+[0-9]+
[^0]+ matches at least one character, and [0-9]+ matches at least 1 character. Both will match 'at least 2 characters' (characters being in scope of the character classes).
Also 23.34.7433.3 doesn't match your regex for the reason I stated above.
And you might try this regex for the purpose you stated:
^(?:[1-9][0-9]{0,2}\.){3}[1-9][0-9]{0,2}$
[1-9][0-9]{0,2} will match up to 3 digits, with a non leading 0.
EDIT: You mentioned in a comment that 0.0.0.0 (single digit zeroes) are to be accepted as well. The modified regex from above would be:
^(?:(?:[1-9][0-9]{0,2}|0)\.){3}(?:[1-9][0-9]{0,2}|0)$

Answer 2

Assuming you want to check an IPv4, I suggest you this pattern:
^(?<nb>2(?>[0-4][0-9]|5[0-5])|1[0-9]{2}|[1-9]?[0-9])(?>\.\g<nb>){3}$
I have defined a named subpattern nb to make the pattern shorter, but if you prefer, you can rewrite all and replace \g<nb>:
^(?>2(?>[0-4][0-9]|5[0-5])|1[0-9]{2}|[1-9]?[0-9])(?>\.(?>2(?>[0-4][0-9]|5[0-5])|1[0-9]{2}|[1-9]?[0-9])){3}$
Numbers greater than 255 are not allowed.
pattern details:
The goal is to describe what is allowed:
numbers with 3 digits that begins with "2" can be followed by a digit in [0-4] and a digit in [0-9] OR by 5 and a digit in [0-5] because it can exceed 255.
numbers with 3 digits that begins with "1" can be followed by any two digits.
any number with 2 digits that doesn't begin with "0"
any number with 1 digit (zero included)
If I add one by one these rules, I obtain
2(?>[0-4][0-9]|5[0-5])
2(?>[0-4][0-9]|5[0-5]) | 1[0-9]{2}
2(?>[0-4][0-9]|5[0-5]) | 1[0-9]{2} | [1-9][0-9]
2(?>[0-4][0-9]|5[0-5]) | 1[0-9]{2} | [1-9][0-9] | [0-9]
Now I have a definition for allowed numbers. I can reduce a little the size of the pattern replacing [1-9][0-9] | [0-9] by [1-9]?[0-9]
Then you only have to add the dot repeat the subpattern four times: x.x.x.x
But since there is only three dots, I write the first number and I repeat 3 times a group that contains a dot and a number:
2(?>[0-4][0-9]|5[0-5])|1[0-9]{2}|[1-9]?[0-9] # the first number
(?>\.2(?>[0-4][0-9]|5[0-5])|1[0-9]{2}|[1-9]?[0-9]){3} # the group repeated 3 times
To be sure that the string doesn't contain anything else that the IP I described, I add anchors for the start of string ^ and for the end of string $, then the string begins and ends with the IP.
To reduce the size of a pattern you can define a named group which allows to reuse the subpattern it contains,
Then you can rewrite the pattern like this:
^
(?<nb> 2(?>[0-4][0-9]|5[0-5])|1[0-9]{2}|[1-9]?[0-9] ) # named group definition
(?> \. \g<nb> ){3} # \g<nb> is the reference to the subpattern named nb
$

Answer 3

[0-9]+ should be [0-9]*
* matches 0 or more.
+ matches 1 or more.
You already have the case [^0] <--- this actually wrong because it will match letters also.
besides that it will match the first character that's NOT zero then at least one number after that.
It should be written as
[1-9][0-9]*
This essentially checks the first letter and sees if its a number that's between 1-9 then the next numbers(0 nums to infinite nums) after that is a number 0-9.
Then this will come out to.
^[1-9][0-9]*\.[1-9][0-9]*\.[1-9][0-9]*\.[1-9][0-9]*$
Edit live on Debuggex
cleaning it up.
^(?:[1-9][0-9]*\.){3}[1-9][0-9]*$
this should work...
^(?:[1-9][0-9]*\.|[0-9])(?:[1-9][0-9]*\.|[0-9])(?:[1-9][0-9]*\.|[0-9])(?:[1-9][0-9]*|[0-9])$
cleaned up.
^(?:(?:[1-9][0-9]*|0)\.){3}(?:[1-9][0-9]*|0)$

Answer 4

Your regex would match ABCDEFG999.FOOBSR888 etc, because [^0] is any character other than a zero, and bith character classes are required by the +.
I think you want this:
^[1-9]\d*(\\.[1-9]\d*){3}$
having replaced various verbose expressions with their equivalent, this is 4 groups of digits each starting with a non-zero.
Actually the problem is far more complicated, because your approach (once corrected) allows 999.999.999.999, which is not a valid IP.

Answer 5

It might be because you need at least two digits between two dots '.'
try using this pattern: ^[^0]+[0-9]*\.[^0]+[0-9]*\.[^0]+[0-9]*\.[^0]+[0-9]*$

Answer 6

to match ip address you should use this pattern:
\b(?:\d{1,3}.){3}\d{1,3}\b
taken from here:
http://www.regular-expressions.info/examples.html