C# Regex Alpahnumeric String

Question

I'd like to check whether a string contains the following Groups. All Strings MUST contain exactly 2 ##

String 1)

##Some_Foo_Text_1##

=> Expected result: String contains 1 Valid group

String 2)

##Some_Foo_Text_2#E+1##

=> Expected result: String contains 1 Valid group

String 3)

##Some_Foo_Text_3#e-1##

=> Expected result: String contains 1 Valid Group

String 4)

##Some_Foo_Text_4##E+1##

=>Expected result: String contains 1 valid Group (##Some_Foo_Text_4##) and 1 invalid Group (E+1##) the invalid Group is discarded

Now i came up with this regex

/([A-Za-z\+\-0-9])+/g

According to Regexr this does not match my string. Could you help me to take care of the ## at the beginning and End?

Answer 1

The below regex will match everything between two occurrences of "##";

(##.*?##)

In your last example string ##Some_Foo_Text_4## becomes a match but not E+1## .

Answer 2

Try this regex

^##(?<content>.+?)##(?!#)

Explanation:

^           // begin of the line
##          // match literaly
(           // to capture group
?<content>  // with name content
.           // any char
+?          // as few times as possible
##          // match literlly
(?!         // assert that 
#           // this regex will not match
)           // end of negative lookahead

Example:

var regex = new Regex("^##(?<content>.+?)##(?!#)");
var input = new[]
{
    "##Some_Foo_Text_21##",
    "##Some_Foo_Text_2#E+1##",
    "##Some_Foo_Text_3#e-1##",
    "##Some_Foo_Text_4##E+1##",
};

foreach (var line in input)
    Console.WriteLine(regex.Match(line).Groups["content"].Value);

Output is

Some_Foo_Text_21

Some_Foo_Text_2#E+1

Some_Foo_Text_3#e-1

Some_Foo_Text_4

Answer 3

This regex will work for all lines posted (?<=##)([A-Za-z0-9_])*(?=#) here is an example https://regexr.com/47aml

Although if it is multiline you need to enable the multiline option to get proper results.