Develop Reference

Regex pattern is not correct

I have the following regex to match this:
U$MichaelU$P#$asdqwe123P#$ - this is correct; the other two are not
U$NameU$P#$PasswordP#$
U$UserU$P#$ad2P#$
A registration is valid when:
The username is surrounded by "U$"
The username needs to be minimum 3 characters long, start with an uppercase letter, followed only by lowercase letters
The password is surrounded by "P#$"
The password needs to start with minimum 5 alphabetical letters (not including digits) and must end with a digit
My regex is
#"^(U\S)([A-z][a-z]{3,})\1(P#\S)([a-z]{5,}[^\d])([\d]+)\3$"
The problem is that it matches the first one but when I submit to the judge it passes first 2 test but the rest it breaks, could you please tell me where is my mistake.

Hello your regex must be
#"^(U\S)([A-Za-z]{3,})\1(P#\S)([A-Za-z0-9]{5,})\3$"
it works for you

Regular expression that Must have at least one letter

I have a case where I am using a queue of regular expressions to filter out specific items in an Observer pattern. The filter will place the values in specific controls based on their values. However 1 of the controls pattern is that it can accept ANY ASCII Character. Let me list the filters in their order with the RegEx
Column Rule Regex
Receiving 7 DIGITS #"^[1-9]([0-9]{6}$)" --->Works
Count 2 digits, no leading 0 #"^[1-9]([0-9]{0,1})$" --->Works
Producer any ASCII char. #".*" --->too broad
MUST contain a letter
Is there a regular expression that will accept any set of ASCII characters, but 1 of them MUST be a letter (upper or lower case)?
#"^(?=.*[A-Za-z])$" -->Didn't work
examples that would need to go into expression
123 red
red
123 red123
red - 123
red

If you want to match the whole rang of ASCII chars you may use
#"^(?=[^A-Za-z]*[A-Za-z])[\x00-\x7F]*$"
If only printable chars are allowed use
#"^(?=[^A-Za-z]*[A-Za-z])[ -~]*$"
Note the (?=[^A-Za-z]*[A-Za-z]) positive lookahead is located right after ^, that is, it is only triggered at the start of a string. It requires an ASCII letter after any 0 or more chars other than an ASCII letter.
Your ^(?=.*[A-Za-z])$ pattern did not work because you wanted to match an empty string (^$) that contains (?=...) at least one ASCII letter ([A-Za-z]) after any 0+ chars other than newline (.*).

You could try [A-Za-z]+.
It matches when there is at least one letter. You want something more specific?

How about
^.*[a-zA-Z]+.*$ ?
Between start and end of line, accept any number of any characters, then at least one a-z/A-Z character, then again any number of any characters.

Regular expression for adding special character to phone number

I have added the following regular expression for validating a mobile phone number:
(^07[1,2,3,4,5,7,8,9][0-9]{7,8}$)
I want to allow the user to enter a # character too and I'm not sure where to fit it in. They may need to enter # character after they have dialed a number, or at the beginning of a number to dial a direct number or an extension.

First, your current regex matches 'numbers' of the format 07,12345678 as well. So you need to change [1,2,3,4,5,7,8,9] to [1-9] (when you have a - between two characters in a character class, it usually means that there's a range)
If you want to accept an optional # character, you can use the ? quantifier which means 0 or 1 times.
^#?07[1-9][0-9]{7,8}#?$
regex101 demo
Except that, as you can see in the demo, it will also match numbers with two hashes; one at the front and one at the end. One option to circumvent this is to use some conditionals (which C# can support).
^(#)?07[1-9][0-9]{7,8}(?(1)|#?)$
regex101 demo
(?(1)|#?) basically means that if the first hash was matched, then nothing more should be matched. Otherwise, if no hash was initially matched, then it can match a hash, if there is one at the end of the number.
In C#, it will be a bit like this:
Regex.Match(myString, #"^(#)?07[1-9][0-9]{7,8}(?(1)|#?)$");
Or you could use a negative lookahead to make sure there's never more than one hash in the number:
^(?!.*#.*#.*$)#?07[1-9][0-9]{7,8}#?$

Regular expression with repetition?

I need a regular expression that accepts only digits and dots, with these conditions:
between digits three must be only one dot '132.632.55'
digits can be repeat in between two dots '.112234563456789.'
the string starts with digits
digits with "." like this '123346547987.' can repeat many times
length of these digits is less than 50 characters
For example: 123456.258469.5467.15546

Given all the information in the question, I think this is the regular expression you need:
^(\d{1,50}\.)*\d{1,50}$
This will:
require that the string begins and ends with a digit
not require that there is a dot in there at all
ensure that each run of digits between dots is no longer than 50 digits
If you need it to have at least one dot, change the * to a +:
^(\d{1,50}\.)+\d{1,50}$

From what I can tell from your requirements, you want something like this:
^(\d{1,50}\.)*\d{1,50}$
That is, from one to 50 digits, optionally preceded by any number of groups of one to 50 digits, each group followed by a fullstop. I can't quite tell if you want something like 1233.456 to be invalid, since your requirement #2 implies that only digit groups between dots can contain repeat digits. In such a case, it'd be much simpler to perform the validation of individual digit groups after the fact.

alphanumeric with at least 1 character

Ok so I finally figured out that I need the following:
So the regular expression has to perform:
alphanumeric
at least 1 letter
must have between 4 and 8 characters (either a letter/digit).
i.e. an alphanumeric string, that must have at least 1 letter, and be between 4-8 in length. (so it can't be just all numbers or just all letters).
can all of this be done in a single regex?

I'm assuming you mean alphaunumeric, at least one letter, and 4 to 8 characters long.
Try this:
(?=.*[a-zA-Z])[a-zA-Z0-9]{4,8}
(?= - we're using a lookahead, so we can check for something without affecting the rest of the match
.*[a-zA-Z] - match for anything followed by a letter, i.e. check we have at least one letter
[a-zA-Z0-9]{4,8} - This will match a letter or a number 4 to 8 times.
However, you say the intention is for "it can't be just all numbers or just all letters", but requirements 1, 2 and 3 don't accomplish this since it's can be all letters and meet all three requirements. It's possible you want this, with an extra lookahead to confirm there's at least one digit:
(?=.*[a-zA-Z])(?=.*[0-9])[a-zA-Z0-9]{4,8}
The use of a-zA-Z isn't very international friendly, so you may be better off using an escape code for "letter" if available in your flavour of Regular Expressions.
Also, I hope this isn't matching for acceptable passwords, as 4 characters probably isn't long enough.

number 2 and 3 seem to contradict. The following will match alphanumeric between 4 and 8:
/[0-9a-zA-Z]{4,8}/

?Regex.IsMatch("sdf", "(?=.+[a-zA-Z])[a-zA-Z0-9]{4,8}")
false
?Regex.IsMatch("sdfd", "(?=.+[a-zA-Z])[a-zA-Z0-9]{4,8}")
true
?Regex.IsMatch("1234", "(?=.+[a-zA-Z])[a-zA-Z0-9]{4,8}")
false
Warning on **.* and .+:
// At least one letter does not match with .*
?Regex.IsMatch("1111", "(?=.*[a-zA-Z])[a-zA-Z0-9]{4,8}")
false
?Regex.IsMatch("1aaa", "(?=.+[a-zA-Z])[a-zA-Z0-9]{4,8}")
true

[a-zA-Z0-9]{4,8}
The first part specifies alphanumeric, and the 2nd part specifies from 4 to 8 characters.

Try: [a-zA-Z0-9]{4,8}