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Regex expression with decimals, one letter and a question mark

I'm trying to make a suvat calculator so one can input decimals, a letter (e.g., S) and a question mark if you do not have a value.
Tests that will be valid include "2.3", "S", "?" but not values like "2.5s", "??", etc (only one type, can't have decimals AND a letter in the same input box)
Is there a regex expression for this? So far I have only got the regex for the decimal number:
^[0-9]\\d*(\\.\\d+)
I did also try a way simpler one but I would like a more developed expression for later on.
[0-9sS.?]

if i got your use case right, then this might work:
^(\?|(\d+\.?\d+)|\S)$
Read it as: The word contains either one question mark,
or a numeric value with propably a dot and numbers behind that
or a single letter
You can try it our here:
https://regex101.com/r/wLGJhJ/1

You can use
#"^(?:[0-9]+(?:\.[0-9]+)?|[A-Za-z?])\z"
Details:
^ - start of string
(?: - start of a non-capturing group:
[0-9]+ - one or more ASCII digits
(?:\.[0-9]+)? - an optional occurrence of . and one or more ASCII digits
| - or
[A-Za-z?] - an ASCII letter or ?char
) - end of the group
\z - the very end of string.
See a .NET regex demo online.

.NET Regex - Specific String containing a changing number

I am new to working with Regexs in C# .NET. Say I have a string as follows...
"Working on log #4"
And within this string we can expect to see the number (4) vary. How can I use a Regex to extract only that number from the string.
I want to make sure that the string matches the first part:
"Working on log #"
And then exctract the integer from it.
Also - I know that I could do this using string.Split(), or .Substring, etc. I just wanted to know how I might use regex's to do this.
Thanks!

"Working on log #(\d+)"
The () create a match group, so you will be able to extract that section.
The \d matches any digit.
The + says "look at the previous token, match it one or more times" so it will make it match one or more digits.
So overall you're capturing a group containing one or more digits, where that group comes after "Working on log #"

RegEx rgx = new RegEx("Working on log #[0-9]"); is the pattern you want to use. The first part is a string literal, [0-9] says that character can be any value 0 through 9. If you allow multiple digits then change it to [0-9]{x} where x is the number of repetitions or [0-9]+ as a + after any character means 1 or more of that character is allowed.
You could also just do string.StartsWith("Working on log #") then split on # and use int.TryParse() with the second value to confirm it is in fact a valid integer.

Try this: ^(?<=Working on log #)\d+$. This only captures the number. No need for a capture group. Remove ^ and $ if this is within a larger string.
^ - start of string
(?<=) - positive lookbehind - ensures what is between = and ) is found before
\d+ - at least one digit
$ - end of string

A capturing group is the solution:
"Working on log #(?<Number>[0-9]+)"
Then you can access the matched groups using the Match.Groups property.

Regular Expressions: Determining if a String is either a number or variable

I am trying to combine two Regular Expression patterns to determine if a String is either a double value or a variable. My restrictions are as follows:
The variable can only begin with an _ or alphabetical letter (A-Z, ignoring case), but it can be followed by zero or more _s, letters, or digits.
Here's what I have so far, but I can't get it to work properly.
String varPattern = #"[a-zA-Z_](?: [a-zA-Z_]|\d)*";
String doublePattern = #"(?: \d+\.\d* | \d*\.\d+ | \d+ ) (?: [eE][\+-]?\d+)?";
String pattern = String.Format("({0}) | ({1})",
varPattern, doublePattern);
Regex.IsMatch(word, varPattern, RegexOptions.IgnoreCase)
It seems that it is capturing both Regular Expression patterns, but I need it to be either/or.
For example, _A2 2 is valid using the code above, but _A2 is invalid.
Some examples of valid variables are as follows:
_X6 , _ , A , Z_2_A
And some examples of invalid variables are as follows:
2_X6 , $2 , T_2$
I guess I just need clarification on the pattern format for the Regular Expression. The format is unclear to me.

As noted, the literal whitespace you've put in your regular expressions is part of the regular expression. You won't get a match unless that same whitespace is in the text being scanned by the regular expression. If you want to use whitespace to make your regex, you'll need to specify RegexOptions.IgnorePatternWhitespace, after that, if you want to match any whitespace, you'll have to do so explicitly, either by specifying \s, \x20, etc.
It should be noted that if you do specify RegexOptions.IgnorePatternWhitespace, you can use Perl-style comments (# to end of line) to document your regular expression (as I've done below). For complex regular expressions, someone 5 years from now — who might be you! — will thank you for the kindness.
Your [presumably intended] patterns are also, I think, more complex than they need be. A regular expression to match the identifier rule you've specified is this:
[a-zA-Z_][a-zA-Z0-9_]*
Broken out into its constituent parts:
[a-zA-Z_] # match an upper- or lower-case letter or an underscore, followed by
[a-zA-Z0-9_]* # zero or more occurences of an upper- or lower-case letter, decimal digit or underscore
A regular expression to match the conventional style of a numeric/floating-point literal is this:
([+-]?[0-9]+)(\.[0-9]+)?([Ee][+-]?[0-9]+)?
Broken out into its constituent parts:
( # a mandatory group that is the integer portion of the value, consisting of
[+-]? # - an optional plus- or minus-sign, followed by
[0-9]+ # - one or more decimal digits
) # followed by
( # an optional group that is the fractional portion of the value, consisting of
\. # - a decimal point, followed by
[0-9]+ # - one or more decimal digits
)? # followed by,
( # an optional group, that is the exponent portion of the value, consisting of
[Ee] # - The upper- or lower-case letter 'E' indicating the start of the exponent, followed by
[+-]? # - an optional plus- or minus-sign, followed by
[0-9]+ # - one or more decimal digits.
)? # Easy!
Note: Some grammars differ as to whether the sign of the value is a unary operator or part
of the value and whether or not a leading + sign is allowed. Grammars also vary as to whether
something like 123245. is valid (e.g., is a decimal point with no fractional digits valid?)
To combine these two regular expression,
First, group each of them with parentheses (you might want to name the containing groups, as I've done):
(?<identifier>[a-zA-Z_][a-zA-Z0-9_]*)
(?<number>[+-]?[0-9]+)(\.[0-9]+)?([Ee][+-]?[0-9]+)?
Next, combine with the alternation operation, |:
(?<identifier>[a-zA-Z_][a-zA-Z0-9_]*)|(?<number>[+-]?[0-9]+)(\.[0-9]+)?([Ee][+-]?[0-9]+)?
Finally, enclose the whole shebang in an #"..." literal and you should be good to go.
That's about all there is to it.

Spaces are not ignored in regular expressions by default, so for each space in your current expressions it is looking for a space in that string. Add the RegexOptions.IgnorePatternWhitespace flag or remove the spaces from your expressions.
You will also want to add some beginning and end of string anchors (^ and $ respectively) so you do not match just part of a string.

You should avoid having spaces in your regular expressions unless you explicitly set IgnorePatterWhiteSpace. To make sure you get only matches on complete words you should include the beginning of line (^) and end of line ($) characters. I would also suggest you build the entire expression pattern instead of using String.Format("({0}) | ({1})", ...) as you have here.
The below should work given your examples:
string pattern = #"(?:^[a-zA-Z_][a-zA-Z_\d]*)|(?:^\d+(?:\.\d+){0,1}(?:[Ee][\+-]\d+){0,1}$)";

Regular expression in C# for Number with Decimal Point & Commas

I need to enter amount in a textbox which allows numbers with decimal point and commas.
What is the regular expression for this?
I used the below
txtInitialAmountGuarDetails.ValidationSettings.RegularExpression.ValidationExpression
= #"^[-+]?\d*[0-9](|.\d*[0-9])(|,\d*[0-9])?$";
But it not working for large numbers like 300,000,000,000,000.

Build it up piecemeal. Given a US locale, a number with these rules has in order:
The string beginning: ^
An optional sign: [+-]?
Up to 3 digits: \d{1,3}
A comma followed by 3 digits, repeated any number of times: (?:,\d{3})*
An optional decimal point and decimal part: (?:[.]\d+)?
The string end: $
Do you have restrictions on the number of digits after the decimal point? Then change the last plus sign to {2} for 2 digits.
So, the regex is:
#"^[+-]?\d{1,3}(?:,\d{3})*(?:[.]\d+)?$"
Or, if you want to explain your work, use the x option and:
#"(?x) # Extended format.
^[+-]? # Optional sign.
\d{1,3} # Initial 1-3 digits.
(?:,\d{3})* # Any number of commas followed by 3 digits.
(?:[.]\d+)?$" # An optional decimal point followed by any number of digits.
But does C# have a locale-dependent validator already?

I have not run it, but you can try it out.
var regexp =/^\s*?([\d\,]+(\.\d{1,2})?|\.\d{1,2})\s*$/;

This works: \d{1,3}(,\d{3})*\.{0,1}(\d{3},)*\d{0,3}
As for the after the comma issue, any choice should be fine. If you go with commas, my regex works. If you do 5 digits then a space just replace the end with (\d{5}\s{1})*\d{0,5}. And ofcourse if you just dont use any deliminator after the decimal you just put \d*

You can try this regex too:
^([+-]?\d{1,3}(?:,\d{1,3})*(?:\.\d+)*)$
Keep in mind . has a specific meaning in regex engine so it is necessary to escape it.
I would also suggest you to not use regex for this task instead look at masked textbox.

try this one:
^([0-9]{3}[,.]|)+[0-9]{0,3}$
let me know if it needs any enhancements...

Discount mask with regex

Is it possible to create a 'dynamic' discount mask that takes % or numbers as discount values? What is the simple way to do this?
the samples of valide input: -25% or 0.25 or -5$ not 0 and two digit after dot

Try
#"(\+|-)?(\d+(\.\d*)?|\.\d+)%?"
It will find:
123.23
12.4%
.34
.34%
45.
45.%
8
7%
34
34%
+2.55%
-1.75%
UPDATE
and with ...
#"(\+|-)?(\d+(,\d{3})*(?!\d)(\.\d*)?|\.\d+)%?"
... you can include thousands separators as well.
I must confess that my second regex expression looks like a cat had walked accross my keyboard. Here the explanation
(\+|-)? optionally ? a plus or a minus sign.
\d+(,\d{3})*(?!\d)(\.\d*)? one or more digits \d+ followed by any number of thousands separators plus three digits (,\d{3})*, not followed by any digit (?!\d) in order to disallow four digits in sequence, optionally followed by a decimal point and any number of digits (\.\d*)?.
|\.\d+ or alternatively a decimal point followed by at least one digit.
%? finally an optional percent sign.

If I understand your question right, you want something like this:
#"^[+-]?(?:\d*\.)?\d+[%$]?$"
That's partly based on your example of -5$. Usually, though, the $ would go in front, so you'd want something like:
#"^(?:\$(?!.*%))?[+-]?(?:\d*\.)?\d+%?$"
That would allow $-5.00, 10, or +20%, but block $5%.
Edit:
Running with Olivier's idea of allowing commas:
#"^(\$(?!.*%))?[+-]?(\d{1,3}((,\d{3})*|\d*))?(\.\d+)?\b%?$"
Expanded to make it easier to understand:
#"^ #Require matching from the beginning of the line
(\$(?!.*%))? #Optionally allow a $ here, but only if there's no % later on.
[+-]? #Optionally allow + or - at the beginning
(
\d{1,3} #Covers the first three numerals
((,\d{3})*|\d*) #Allow numbers in 1,234,567 format, or simply a long string of numerals with no commas
)? #Allow for a decimal with no leading digits
(\.\d+)? #Optionally allow a period, but only with numerals behind it
\b #Word break (a sneaky way to require at least one numeral before this position, thus preventing an empty string)
%? #Optionally allow %
$" #End of line