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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.

Regex to match if string is exactly as defined

How can I check that the string is in correct format. I want the string to compare and pass only if matches exactly. Following are the correct formats :
0.#
0.##
0.###
0.####
0.#####
The hash (#) after the dot (.) can be upto 10 characters but it should only have 0.# nothing else is allowed.
Can someone please guide me how can I validate a string of this type ?

Im Regular Expression the carret (^) represent start-of-line and the ($) represents end-of-line (or before newline).
A regex with an exact match is just what you want enclosed by ^ and $. But you must ensure that special regular expression characters are quoted. For example the regex
^Hello World$
would match exactly on the String "Hello World" and nothing else.
You also can use numbers directly. You need to escape the dot "." as a dot in a regular expression means any character except newline. You escape a character by adding a backslash.
Next you should know about quantifiers. The usually ones are
-> 0 or many
-> 1 or many
{n} -> exactly n times
{n,} -> at least n times
{n,m} -> n to m times
So you can write:
^0\.#{1,10}$
If you use a normal string in C# with quotations (") you must use two backslashes
^0\\.#{1,10}$

Regular expressions to extract punctuations/characters

Hi guys I need a regex that only extracts punctuations/characters.
I have this so far :
[._^%$#!~#,-]+
but this works if there is at least 1 punctuations and still allows for any other char (digit or letter)
I need to to only allow punctuations/characters

Try this:
^[\p{S}\p{P}]+$
\p{S} matches any symbol character, and \p{P} matches any punctuation.
Note that your pattern will not match all the symbols and punctuations not present in the list.

Try anchoring the regex to the start and the end of the string (unless you're using Multiline matching) - i.e. ^ at the beginning and $ at the end:
^[._^%$#!~#,-]+$
Note - this does not endorse your actual pattern (I can't say whether this is matching all the 'special characters' you're talking about, but it will make it so that the entire string must be all 'special'.

[^a-zA-Z0-9]* u can try something like this. Should NOT accept those chars, cba to writte all beside chars beside one u typed.

\W
Matches any character that is not a word character (alphanumeric & underscore).

regular expression in c#

I am trying to validate a texbox to allow numbers and letter(s) but not letters alone only e.g. 13492M
I am using C# regular expressions.

^[A-Za-z]*\d[A-Za-z\d]*$ should do it. (Possibly some letters, then a digit, then any more letters or digits.)
(Edited to add start/end matches.)

use maskedTextBox, it use a property "mask" to validate with the Expression that you want. So you only add the RegEx to your maskedTextBox and you don't have to validate everytime in your code (it will check against your RegEx automatically)

How about this:
([0-9]+[a-zA-Z]+ | [a-zA-Z]+[0-9]+)[a-zA-Z0-9]*
(Numbers first and then alphabets OR Alphabets first then numbers) atleast once or more then both alphabets and numbers which is optional

This Regex should work fine:
^[A-Za-z]*[0-9]+[A-Za-z]*$
This regex will allow numbers or letters+numbers. Just letters will fail.

Simply,
Pattern = "^[a-zA-Z0-9]*[0-9]+[a-zA-Z0-9]*$"
Details :
Start. ^
Zero or more mixed alphanumerical. [a-zA-Z0-9]*
One or more numerical. [0-9]+
Zero or more mixed alphanumerical. [a-zA-Z0-9]*
End. $

Regexp Remove any non alphanumeric, but leave some special characters in one expression

I have this code that replaces all non alphanumeric characters with "-" char.
return Regex.Replace(strIn, #"[\W|_]+", "-", RegexOptions.IgnorePatternWhitespace | RegexOptions.IgnoreCase);
but I need to change it to allow pass some special characters (one or more) for example: #,*,%
how to change this regular expression?

Use
[^\p{L}\p{N}#*%]+
This matches one or more characters that are neither letters nor digits nor any of #, * or %.

Another option, you can use charcter class subtractioninfo, for example to remove # from the character class:
[\W_-[#]]+
Just add other accepted special chars after the #. Live example here: http://rextester.com/rundotnet?code=YFQ40277

How about this one:
[^a-zA-Z0-9#*%]+
If you are using unicode you can do (as Tim's answer):
[^\p{L}\p{N}#*%]+

Use this.
([^\w#*%]|_)
Add any other special characters after the %.
It is basically saying, match any character that is not (^) a word character(\w), #, * or % OR match _.

It seems this way is the best solution for you
#"(?!.*[^\w#*%])"

You can use set subtraction for that:
#"[\W_-[#*%]]+"
This matches the set of all non-word characters and the underscore, minus the set of #, * and %.
Note that you don't have to use | for "or" in a character class, since that's implied. In fact, the | in your regex just matches |.
Note also that in .NET, \w matches a few other "connector punctuation" characters besides the underscore. If you want to match the other characters too, you can use
#"[\W\p{Pc}-[#*%]]+"