Develop Reference

C# Regex contain []

I am working on a regex to validate that a string input contains letters with in a length of 6 to 20 and inside the string exists square brackets with an integer.
Something like:
ABC[15]WHATEVER
I get that it would be better to check for the string length without the the use of regex, but still wandering for the brackets with the integer.
What I managed to do:
\[([0-9\-]+)]
which is working by testing through regex tester .net
Is this the appropriate solution though?
Any help is welcome

I'd go with
^(?=[\w\[\]]{6,20}$)[A-Z]+\[\d+\][A-Z]+$
see this working example on Regex101
Explanation
^ Begin of string
(?=[\w\[\]]{6,20}$) Followed by a string between 6 and 20 characters long ({6,20}) containing only alphanumeric characters (\w) and brackets (\[\]), followed by the end of the string ($)
[A-Z]+\[\d+\][A-Z]+ the actual pattern - 1 or more digits in brackets surrounded by characters
$ end of string

I'd comment but I don't have the rep. Consider this an expansion on Jeroen's comment.
A length check and regex to match the bracketed integer won't necessarily guarantee a legitimate string. What if there are two bracketed integers? What if the 'letters' prior to the brackets contain an invalid character? I'd recommend a string length check followed by something like:
^[a-zA-Z]+\[[0-9]+\][a-zA-Z]+$
Which will further constrain the match. Add capturing groups as needed.

I think you should use string.Length and regex (^[a-zA-Z]+\[-?\d+\][a-zA-Z]+$) to test the completeness.

Writing a proper regex to allow number and only combinations of letters and numbers mixed up

I have a string example which looks like this:
51925120851209567
The length of the string and numbers may vary, however I want to only enable the string to contain just either numbers, or for it to be a combination of letters and numbers. For example a valid one would be something like this:
B0031Y4M8S // contains combination of letters and numbers without white space
Invalid regex would be:
Does not apply // this one contains white spaces and has only letters
To summarize things up, the regex should allow only these combinations:
51925120851209567 // contains only numbers and is valid
B0031Y4M8S // contains combination of numbers and letters and is valid as well
Everything else is invalid...
The current solution that I have covers only for the string to be a set of integers and nothing else... However I'm not really sure how to filter out combination of numbers and letters without white spaces and special charachters to be valid as well for the regex?
Regex regex = new Regex("^[0-9]+$");
if (regex.IsMatch(parameter))
{
// allow if statement to pass if the regex matches
}
Can someone help me out ?

You may use
^(?![A-Za-z]+$)[0-9A-Za-z]+$
It matches 1+ alphanumeric chars but will fail a match if all string consists of just letters.
Details
^ - start of a string
(?![A-Za-z]+$) - a negative lookahead that fails the match if there are 1+ ASCII letters followed with the end of string immediately to the right of the current location
[0-9A-Za-z]+ - 1+ ASCII letters
$ - end of string.
See the regex demo.

#The fourth bird's answer will almost get you there. I'm no regex expert, but an easy way to get you what you want would be to use:
Regex regex = new Regex("^[a-zA-Z0-9]+$");
This will get you the first level of exclusion. If it passes that, then check with:
Regex regex = new Regex("^[a-zA-Z]+$");
If it matches that, then you know it's only alphabetical characters and you can skip it. I'm sure there's a better way to code golf this one out, but this should work for now if you're in a crunch.

Pattern matching for swedish character

I need a help regarding regular expression.
I have to match string like this:
âãa34dc
Pattern that i have used:
\s*[a-zA-Z]+[a-zA-Z_0-9]*\s
but this pattern is not good enough to identify this kind of string e.g. âãa34dc
P.S. âã these are swedish character.
Please help me for find out correct pattern for this kind of string.

Do you actually want to restrict it to Swedish characters? In other words, should a German character not match? If so, then you'll probably have to enumerate the whole alphabet, and include that.
If what you really want is to match every alphabetic character, use the regular expression terms for matching all letters.
\w matches any word character, but that includes numbers & some punctuation. That's close, but not exactly what you want for your second term.
For the first term, where you don't want to include numbers, specifying that the character should be a Unicode 'letter' class will work. \p{L} specifies all Unicode characters that are a letter. This includes [a-zA-Z], and all the Swedish characters, and German, and Russian, etc.
Therefore, I think this regular expression is what you want:
\s*[\p{L}][\p{L}_0-9]*\s
If you want to include digits from other character sets, and some other punctuation, then you can use [\w]* for the second term.

please give a set of rules.
according to your question :
[X-Ya-zA-Z]{3}[0-9]{2}[a-zA-Z]{2}
Replace X with the first swedish letter
Replace Y with the last swedish letter

John Machin provides a great answer for this. Adapting his pattern, what you need is probably something similar to: \s*[^\W\d_]\w*\s*
P.S. I removed the + quantifier from your first part. Any subsequent letters would be matched by the subsequent quantified \w.

Regex Expression Only Numbers and Characters

I created the following regex expression for my C# file. Bascily I want the user's input to only be regular characters (A-Z lower or upper) and numbers. (spaces or symbols ).
[a-zA-Z0-9]
For some reason it only fails when its a symbol on its own. if theres characters mixed with it then the expression passes.
I can show you my code of how I implment it but I think its my expression.
Thanks!

The problem is that it can match anywhere. You need anchors:
^[a-zA-Z0-9]+\z
^ matches the start of a string, and \z matches the end of a string.
(Note: in .NET regex, $ matches the end of a string with an optional newline.)

This is because it will match any character in the string you need the following.
Forces it to match the entire string not just part of it
^[0-9a-zA-Z]*$

That regex will match every single alphanumeric character in the string as separate matches.
If you want to make sure the whole string the user entered only has alphanumeric characters you need to do something like:
^[a-zA-Z0-9]+$

Are you making sure to check the whole string? That is are you using an expression like
^[a-zA-Z0-9]*$
where ^ means the start of the string and $ means the end of the string?

Regex - Match a string only when it contains any alphabetic characters

example strings
785*()&!~`a
##$%$~2343
455frt&*&*
i want to capture the first and the third but not the second since it doesnt contain any alphabet character plz help

In fact, I think [a-zA-Z] might suffice to match your strings.
To capture the whole thing, try: ^.*[a-zA-Z].*$

Here is one possible way:
.*[a-zA-Z]+

You should maybe clarify a bit what you mean by 'catpuring': do you want the whole string of just the ascii bits?
Also, you don't say if it should match just plain Roman alphabet (A to Z) or if it should also match Unicode chars to match strings in other languages.
If you just need to test your string, in C# you would do:
bool matching = Regex.IsMatch(myString, "[a-zA-Z]");
You wouldn't need anything else, since just one letter anywhere in the myString string will match (according to your definition).

This is my favorite RegEx testing site: Javascript Regexp Tester and Cheat Sheet

If you want to match all letters (including non-ascii ones), use p{L} instead of [a-zA-Z]. See Unicode categories.