Develop Reference

My regular expresiion is not working in C#

I need a regex that checks if a password contains at least one Lowercase letter, at least one Upper case letter, at least two numbers and at least one of(_*&$). This is a MVC project.
This what i have
[RegularExpression(#"(?=\.\*\\d{2})(?=\./*[a-z])(?=\.\*[A-Z])(?=.*[_*&$])", ErrorMessage = "The password must contain at least 1 letter, 2 digits and a special symbol (_*&$)")]

There are a lot of issues with the current regex:
You escaped . and *, and now they denote literal . and * chars, while you wanted to use them as special regex metacharacters
To match at least two digits, you can't just use a \d{2} pattern because it does not match non-consecutive digits, you need \d.*\d or a more efficient (?:\D*\d){2}
You are only using lookaheads, non-consuming patterns, but the RegularExpressionAttribute requires a full string to match the pattern.
Thus, you need
#"^(?=(?:\D*\d){2})(?=[^a-z]*[a-z])(?=[^A-Z]*[A-Z])(?=[^_*&$]*[_*&$]).*"
Details
^ - start of string
(?=(?:\D*\d){2}) - two not necessarily consecutive digits
(?=[^a-z]*[a-z]) - at least one lowercase ASCII letter
(?=[^A-Z]*[A-Z]) - at least one uppercase ASCII letter
(?=[^_*&$]*[_*&$]) - at least one special char from the _*&$ set
.* - the whole string (with no line breaks) (it gets consumed).

Regex for first name

I am quite new to regex thing and need regex for first name which satisfies following conditions:
First Name must contain letters only. It may contain spaces, hyphens, or apostrophes.
It must begin with letters.
All other characters and numbers are not valid.
Special characters ‘ and – cannot be together (e.g. John’-s is not allowed)
An alphabet should be present before and after the special characters ‘ and – (e.g. John ‘s is not allowed)
Two consecutive spaces are not allowed (e.g. Annia St is not allowed)
Can anyone help? I tried this ^([a-z]+['-]?[ ]?|[a-z]+['-]?)*?[a-z]$ but it's not working as expected.

Regexes are notoriously difficult to write and maintain.
One technique that I've used over the years is to annotate my regexes by using named capture groups. It's not perfect, but can greatly help with the readability and maintainability of your regex.
Here is a regex that meets your requirements.
^(?<firstchar>(?=[A-Za-z]))((?<alphachars>[A-Za-z])|(?<specialchars>[A-Za-z]['-](?=[A-Za-z]))|(?<spaces> (?=[A-Za-z])))*$
It is split down into the following parts:
1) (?<firstchar>(?=[A-Za-z])) This ensures the first character is an alpha character, upper or lowercase.
2) (?<alphachars>[A-Za-z]) We allow more alpha chars.
3) (?<specialchars>[A-Za-z]['-](?=[A-Za-z])) We allow special characters, but only with an alpha character before and after.
4) (?<spaces> (?=[A-Za-z])) We allow spaces, but only one space, which must be followed by alpha characters.
You should use a testing tool when writing regexes, I'd recommend https://regex101.com/
You can see from the screenshot below how this regex performs.
Take the regex I've given you, run it in https://regex101.com/ with samples you'd like to match against, and tweak it to fit your requirements. Hopefully I've given you enough information to be self sufficient in customising it to your needs.
You can use this link to run the regex https://regex101.com/r/O2wFfi/1/
Edit
I've updated to address the issue in your comment, rather than just give you the code, I will explain the problem and how I fixed it.
For your example "Sam D'Joe", if we run the original regex, the following happens.
^(?<firstchar>[A-Za-z])((?<alphachars>[A-Za-z])|(?<specialchars>[A-Za-z]['-][A-Za-z])|(?<spaces> [A-Za-z]))*$
1) ^ matches the start of the string
2) (?<firstchar>[A-Za-z]) matches the first character
3) (?<alphachars>[A-Za-z]) matches every character up to the space
4) (?<spaces> [A-Za-z]) matches the space and the subsequent alpha char
Matches consume the characters that they match
This is where we run into a problem. Our "specialchars" part of the regex matches an alpha char, our special char and then another alpha char ((?<specialchars>[A-Za-z]['-](?=[A-Za-z]))).
The thing you need to know about regexes, is each time you match a character, that character is then consumed. We've already matched the alpha char before the special character, so our regex will never match.
Each step actually looks like this:
1) ^ matches the start of the string
2) (?<firstchar>[A-Za-z]) matches the first character
3) (?<alphachars>[A-Za-z]) matches every character up to the space
4) (?<spaces> [A-Za-z]) matches the space and the subsequent alpha char
and then we're left with the following
We cannot match this, because one of our rules is "An alphabet should be present before and after the special characters ‘ and –".
Lookahead
Regex has a concept called "lookahead". A lookahead allows you to match a character without consuming it!
The syntax for a lookahead is ?= followed by what you want to match. E.g. ?=[A-Z] would look ahead for a single character that is an uppercase letter.
We can fix our regex, by using lookaheads.
1) ^ matches the start of the string
2) (?<firstchar>[A-Za-z]) matches the first character
3) (?<alphachars>[A-Za-z]) matches every character up to the space
4) We now change our "spaces" regex, to lookahead to the alpha char, so we don't consume it. We change (?<spaces> [A-Za-z]) to (?<spaces> ?=[A-Za-z]). This matches the space and looks ahead to the subsequent alpha char, but doesn't consume it.
5) (?<specialchars>[A-Za-z]['-][A-Za-z]) matches the alpha char, the special char, and the subsequent alpha char.
6) We use a wildcard to repeat matching our previous 3 rules multiple times, and we match until the end of the line.
I also added lookaheads to the "firstchar", "specialchars" and "spaces" capture groups, I've bolded the changes below.
^(?<firstchar>(?=[A-Za-z]))((?<alphachars>[A-Za-z])|(?<specialchars>[A-Za-z]['-](?=[A-Za-z]))|(?<spaces> (?=[A-Za-z])))*$

This short regex should do it ^([a-zA-Z]+?)([-\s'][a-zA-Z]+)*?$ ,
([a-zA-Z]+?) - Means the String should start with alphabets.
([-\s'][a-zA-Z]+)*? - Means the string must have hyphen,space or apostrophe followed by alphabets.
^ and $ - denote start and end of string
Here's the link to regex demo.

Try this one
^[^- '](?=(?![A-Z]?[A-Z]))(?=(?![a-z]+[A-Z]))(?=(?!.*[A-Z][A-Z]))(?=(?!.*[- '][- '.]))(?=(?!.*[.][-'.]))[A-Za-z- '.]{2,}$
Demo

Regex to restrict special characters in the beginning of an email address

PFB the regex. I want to make sure that the regex should not contain any special character just after # and just before. In-between it can allow any combination.
The regex I have now:
#"^[^\W_](?:[\w.-]*[^\W_])?#(([a-zA-Z0-9]+)(\.))([a-zA-Z]{2,3}|[0-9]{1,3})(\]?)$"))"
For example, the regex should not match
abc#.sj.com
abc#-.sj-.com
SSDFF-SAF#-_.SAVAVSAV-_.IP

Since you consider _ special, I'd recommend using [^\W_] at the beginning and then rearrange the starting part a bit. To prevent a special char before a #, just make sure there is a letter or digit there. I also recommend to remove redundant capturing groups/convert them into non-capturing:
#"^[^\W_](?:[\w.-]*[^\W_])?#(?:\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.|(?:[\w-]+\.)+)(?:[a-zA-Z]{2,3}|[0-9]{1,3})\]?$"
Here is a demo of how this regex matches now.
The [^\W_](?:[\w.-]*[^\W_])? matches:
[^\W_] - a digit or a letter only
(?:[\w.-]*[^\W_])? - a 1 or 0 occurrences of:
[\w.-]* - 0+ letters, digits, _, . and -
[^\W_] - a digit or a letter only

Change the initial [\w-\.]+ for [A-Za-z0-9\-\.]+.
Note that this excludes many acceptable email addresses.
Update
As pointed out, [A-Za-z0-9] is not an exact translation of \w. However, you appear to have a specific definition as to what you consider special characters and so it is probably easier for you to define within the square brackets what you class as allowable.

Regex failing in max length

I want regex which will allow following format
1234567-8
123456B
Now here if user enter second pattern then he should be lock to enter maximum 7 characters so
1234568B
123456V1
this becomes invalid
I have tried
[0-9]{7}-[0-9]|[[0-9]{6}[A-z]{1}]{7,7}
but this fails

For the sample input you provided, you may use ^([0-9]{7}-[0-9]|[0-9]{6}[A-Za-z])$.
A bit more contracted version: ^[0-9]{6}(?:[0-9]-[0-9]|[A-Za-z])$.
Note that 1234567-8 has 7 digits and a hyphen followed with a digit, so the whole string length cannot be limited to just 7 characters all in all.
In .NET and almost all other regex flavors [A-z] is a mistake, as it can match more than just letters.
Placing a quantifier {1} into a character class makes it a simple symbol combination, so [{1}] matches either { or 1 or }.
The {7,7} (={7}) will not limit the whole string length to 7, as you do not have anchors (^ and $) around the expression and you "ruined" the preceding quantifiers by putting them into a character class.

I think this is what you need:
^(\d{7}-\d|\d{6}[A-Z])$
7 digits, dash, digit OR 6 digits, 1 large latin letter.

^\d{6}(?:\d-\d|[A-Z])$
It can satisfy well with 2 your above formats
1234567-8
123456B

Regex to match more than one word

I have an ASP.NET MVC application containing a form field called 'First/last name'. I need to add some basic validation to ensure people enter at least two words. It doesn't need to be totally comprehensive in checking word length etc, we essentially just need to prevent people from entering just their first name which is what's happening currently. I don't want to limit to just alphabetic characters as some names include punctuation. I just want to ensure that people have entered at least two words separated by a space.
I have the following regex currently:
[RegularExpression(#"^((\b[a-zA-Z]{2,40}\b)\s*){2,}$", ErrorMessage = "Invalid first/last name")]
This works to an extent (it checks for 2 words) but it's invalid if punctuation is entered, which isn't what I'm looking for.
Could anyone suggest how to modify the above so that it doesn't matter if punctuation is used in the words? I'm not good with the regular expression syntax, hence asking here.
Thanks.

You want two words, so at least one space between them, and beyond that you want to allow everything else (e.g., punctuation). So keep it simple:
\w.*\s.*\w
Or if you must anchor it to start and end:
^.*\w.*\s.*\w.*$
These will match, for example, D' Addario (but not D'Artagnan by itself, since it counts as one word by the space criterion).

Maybe just:
#"\w\s\w"
word white space word

Hi you can use this regex for validation
'^[a-zA-Z0-9]+ {1}[a-zA-Z0-9]+$`'
Demo http://rubular.com/r/YN8eFa1yFE

If you just want to allow a sequence of non-whitespace characters followed by 1 or more sequences of whitespace characters followed by non-whitespace characters, you can use
^\s*\S+(?:\s+\S+)+\s*$
See regex demo
It won't accept just First or First .
Regex breakdown:
^ - start of string
\s* - zero or more whitespace
\S+ - 1 or more non-whitespace symbols
(?:\s+\S+)+ - 1 or more sequences of ...
\s+ - 1 or more whitespace sequences (remove + to allow only 1 whitespace between words)
\S+ - 1 or more non-whitespace symbols
\s* - zero or more whitespace
$ - end of string