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Make input field validate a number range and a single character using regular expressions

This is a commonly asked question. But I am not getting the answer that I need. I had a look at MVC Validation make RegularExpression numeric only on string field (among many) for reference but I am not quite there yet.
I have an html5 input field bound to an MVC property. I want to make use of the [RegularExpression()] attribute but I am not getting the output that I need. I need my input to only take the following:
A single number between 0 and 7, or the % character
I tried the following:
[RegularExpression("^[0-7][%]")].
Where am I going wrong?

If you want to use range from 0 to 7. (Only single digit at a time) then use [0-7] and for % use [%]. To concatenate both the requirements(For or condition) use | in between two square brackets.
Your regular expression would be look like this
[RegularExpression("^[0-7]|[%]")]
^ at the starting of regular expressions, suggests that your string should starts with the number.
$ mentioned by #Stephen in comments states that your % must occurs at the end of string or expression.
Now its up to you, if you want string should be in D% format, where D is for digit and %, then use ^ in the beginning and $ at the end of regex. e.g. 5%
if this is not the case, then you can remove ^ and $ from the regex e.g %

Capture Text Surrounding Regex Match .NET

I am building an application, and I have a requirement to capture characters before and after matches. This seems to work okay, except when there are multiple matches within the surrounding capture.
Regex:
.{0,10}(?=abc)
This should capture up to 10 characters before the string "abc" is found.
The issue comes up if there is a recurrence of the match in the preceding text:
"qqqqabcabcqqq"
With the above text, I would expect two captures:
qqqq (the 4 characters before the first abc occurrence)
qqqqabc (the 7 characters before the second abc occurrence)
I am not, however getting these matches. The only match I get is:
qqqqabc
I am certain that I am missing something, but I am not sure what. I believe that my regex is somehow being too greedy, and so it is overlooking the first match in favor of the larger, second one. Here is what I need:
I need a regex that:
1. Is for .NET
2. Looks within a string for X characters before an exact match on string S.
3. Includes any secondary match on S (call S') that is found within X characters before S
4. does not care in the slightest what these characters are.
I assure you, I tried looking for similar answers but I wasn't able to find anything that directly answers this question (which has been plaguing me for two days. Yes, I have to use regular expression). As for Regex flavor, I am working in .NET.
Thank you so much for any help.

Here it is:
(?<=(?<CharsBefore>.{0,10}))(?=abc)
Took me a while to remember that .NET allows positive lookbehinds with variability.
Regex test
Demo in C#
I changed the way your initial version worked a bit.
Hope it helps!
PS: I've named the group, but you are obviously free to keep it nameless and work with numbered groups if you want a less cluttered regex, like so:
(?<=(.{0,10}))(?=abc)

What Regex would I use to match file names with the format 'number-text-text'?

I have code that searches a folder that contains SQL patch files. I want to add file names to an array if they match the following name format:
number-text-text.sql
What Regex would I use to match number-text-text.sql?
Would it be possible to make the Regex match file names where the number part of the file name is between two numbers? If so what would be the Regex syntax for this?

The following regex make it halfway there:
\d+-[a-zA-Z]+-[a-zA-Z]+\.sql
Regarding to match in a specific range it gets trickier as regex doesn't have a simple way to handle ranges. To limit the match to a filename with a number between 2 and 13 this is the regex:
([2-9]|1[0-3])-[a-zA-Z]+-[a-zA-Z]+\.sql

Your regular expression should be:
(\d+)-[a-zA-Z]+-[a-zA-Z]+\.sql
You would then use the first captured group to check if your number is between the two numbers you desire. Don't try to check if a number is within a range with a regular expression; do it in two steps. Your code will be much clearer for it.

How about:
\d+-[^-]+-[^-]+\.sql
Edit: You want just letters, so here it is without specific ranges.
\d+-[a-z]+-[a-z]+\.sql - You'll also want to use the i flag, not sure how that's done in c#, but here it is in js/perl:
/\d+-[a-z]+-[a-z]+\.sql/i
Ranges are more difficult. Here's an example of how to match 0-255:
([0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])
So to match (0-255)-text-text.sql, you'd have this:
/^(?:[0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])-[a-z]+-[a-z]+\.sql/i
(I put the digits in a non-capturing group and matched from the beginning of the string to prevent partial matches on the number and in case you're expecting numbered groups or something).
Basically every time you need another digit of possibility, you'll need to add a new condition inside this case. The smaller the digit you'd like to match, the more cases you'll need as well. What is your desired min/max? AFAIK there's not a simple way to do this dynamically (although I'd love for someone to show me I'm wrong about that).
The simplest way to get around this would be to simply capture the digits, and use native syntax to see if it's in your range. Example in js:
var match = filename.match(/(\d+)-[a-z]+-[a-z]+\.sql/i);
if(match && match[1] < maximumNumber && match[1] > minimumNumber){
doStuff();
}

This should work:
select '4-dfsg-asdfg.sql' ~ E'^[0-9]+-[a-zA-Z]+-[a-zA-Z]+\\.sql$'
This restricts the TEXT to simple ASCII characters. May or may not be what you want.
This is tested in PostgreSQL. Regular expression flavors differ a lot between implementations. You probably know that?
Anchors at begin ^ and end $ are optional, depending how you are going to do it.

.NET Regular expression which check length and non-alphanumeric characters

I need Regexp to validate string has minimum length 6 and it is contains at least one non-alphanumeric character e.g: "eN%{S$u)", "h9YI!>4j", "{9YI!;4j", "eN%{S$usdf)", "dfh9YI!>4j", "ghffg{9YI!;4j".
This one is working well ^.*(?=.{6,})(?=.*\\d).*$" but in cases when string does not contain any numbers(e.g "eN%{S$u)") it is not working.

^(?=.{6})(.*[^0-9a-zA-Z].*)$
We use positive lookahead to assure there are at least 6 characters. Then we match the pattern that looks for at least one non-alphanumeric character ([^0-9a-zA-Z]). The .*'s match any number of any characters around this one non-alphanumeric character, but by the time we've reached here we've already checked that we're matching at least 6.
^.*(?=.{6,})(?=.*\\d).*$"
is the regex you tried. Here are some suggestions:
You don't need to match more than 6 characters in the lookahead. Matching only 6 here does no restrict the rest of the regular expression from matching more than 6.
\d matches a digit, and (?=.*\\d) is a lookahead for one of them. This is why you are experiencing the problems you mentioned with strings like eN%{S$u).
Even if the point above wasn't incorrect and the regular expression here was correct, you can combine the second lookahead with the .* that follows by just using .*\\d.*.

marcog's answer is pretty good, but I'd do it the other way around so that it's easier to add even more conditions (such as having at least one digit or whatever), and I'd use lazy quantifiers because they are cheaper for certain patterns:
^(?=.*?[^0-9a-zA-Z]).{6}
So if you were to add the mentioned additional condition, it would be like this:
^(?=.*?[^0-9a-zA-Z])(?=.*?[0-9]).{6}
As you can see, this pattern is easily extensible. Note that is is designed to be used for checking matches only, its capture is not useful.

Keep it easy.
// long enough and contains something not digit or a-z
x.Length >= 6 && Regex.IsMatch(x, #"[^\da-zA-Z]")
Happy coding.
Edit, pure "regular expression":
This first asserts there are 6 letters of anything in the look-ahead, and then ensures that within the look-ahead there is something that is not alpha-numeric (it will "throw away" up to the first 5 characters trying to match).
(?=.{6}).{0,5}[^\da-zA-Z]

What about that(fixed): ^(?=.{6})(.*[^\w].*)$
Check this out http://www.ultrapico.com/Expresso.htm it is cool tool which could help you a lot in Regexps learning.

How Can I Check If a C# Regular Expression Is Trying to Match 1-(and-only-1)-Character Strings?

Maybe this is a very rare (or even dumb) question, but I do need it in my app.
How can I check if a C# regular expression is trying to match 1-character strings?
That means, I only allow the users to search 1-character strings. If the user is trying to search multi-character strings, an error message will be displaying to the users.
Did I make myself clear?
Thanks.
Peter
P.S.: I saw an answer about calculating the final matched strings' length, but for some unknown reason, the answer is gone.
I thought it for a while, I think calculating the final matched strings length is okay, though it's gonna be kind of slow.
Yet, the original question is very rare and tedious.

a regexp would be .{1}
This will allow any char though. if you only want alpanumeric then you can use [a-z0-9]{1} or shorthand /w{1}
Another option its to limit the number of chars a user can type in an input field. set a maxlength on it.
Yet another option is to save the forms input field to a char and not a string although you may need some handling around this to prevent errors.
Why not use maxlength and save to a char.

You can look for unescaped *, +, {}, ? etc. and count the number of characters (don't forget to flatten the [] as one character).
Basically you have to parse your regex.

Instead of validating the regular expression, which could be complicated, you could apply it only on single characters instead of the whole string.
If this is not possible, you may want to limit the possibilities of regular expression to some certain features. For instance the user can only enter characters to match or characters to exclude. Then you build up the regex in your code.
eg:
ABC matches [ABC]
^ABC matches [^ABC]
A-Z matches [A-Z]
# matches [0-9]
\w matches \w
AB#x-z matches [AB]|[0-9]|[x-z]|\w
which cases do you need to support?
This would be somewhat easy to parse and validate.