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Conditional match without false force a match?

I'm using the following regex in c# to match some input cases:
^
(?<entry>[#])?
(?(entry)(?<id>\w+))
(?<value>.*)
$
The options are ignoring pattern whitespaces.
My input looks as follows:
hello
#world
[xxx]
This all can be tested here: DEMO
My problem is that this regex will not match the last line. Why?
What I'm trying to do is to check for an entry character. If it's there I force an identifier by \w+. The rest of the input should be captured in the last group.
This is a simplyfied regex and simplyfied input.
The problem can be fixed if I change the id regex to something like (?(entry)(?<id>\w+)|), (?(entry)(?<id>\w+))? or (?(entry)(?<id>\w+)?).
I try to understand why the conditional group doesn't match as stated in original regex.
I'm firm in regex and know that the regex can be simplyfied to ^(\#(?<id>\w+))?(?<value>.*)$ to match my needs. But the real regex contains two more optional groups:
^
(?<entry>[#])?
(\?\:)?
(\(\?(?:\w+(?:-\w+)?|-\w+)\))?
(?(entry)(?<id>\w+))
(?<value>.*)
$
That's the reason why I'm trying to use a conditional match.
UPDATE 10/12/2018
I tested a little arround it. I found the following regex that should match on every input, even an empty one - but it doesn't:
(?(a)a).*
DEMO
I'm of the opinion that this is a bug in .net regex and reported it to microsoft: See here for more information

There is no error in the regex parser, but in one's usage of the . wildcard specifier. The . specifier will consume all characters, wait for it, except the linefeed character \n. (See Character Classes in Regular Expressions "the any character" .])
If you want your regex to work you need to consume all characters including the linefeed and that can be done by specify the option SingleLine. Which to paraphrase what is said
Singline tells the parser to handle the . to match all characters including the \n.
Why does it still fail when not in singleline mode for the other lines are consumed? That is because the final match actually places the current position at the \n and the only option (as specified is use) is the [.*]; which as we mentioned cannot consume it, hence stops the parser. Also the $ will lock in the operations at this point.
Let me demonstrate what is happening by a tool I have created which illustrates the issue. In the tool the upper left corner is what we see of the example text. Below that is what the parser sees with \r\n characters represented by ↵¶ respectively. Included in that pane is what happens to be matched at the time in yellow boxes enclosing the match. The middle box is the actual pattern and the final right side box shows the match results in detail by listening out the return structures and also showing the white space as mentioned.
Notice the second match (as index 1) has world in group capture id and value as ↵.
I surmise your token processor isn't getting what you want in the proper groups and because one doesn't actually see the successful match of value as the \r, it is overlooked.
Let us turn on Singline and see what happens.
Now everything is consumed, but there is a different problem. :-)

Using Visual Studio 2013 regular expression find - how to invalidate multiple prefixes

I'm trying to find the "symbol" all in a bunch of C#, XML, and JS files. My project is huge and doing a naive search for "all" results in over 8,000 lines found so I'm trying to eliminate some of them.
For example, I don't want to match on "call" or "balloon" or "Balloon" (those are UX element styles.
Looking at the using Regular Expressions MSDN page (http://msdn.microsoft.com/en-us/library/vstudio/2k3te2cs%28v=vs.110%29.aspx) I found out how to invalidate on one of those but I can't figure out how to do it on multiple and make it case-insensitive.
I started off using:
(?!c)all
And that filtered out call and things like that but I can't get one to filter out multiple to work.
(?!b|c)all
Is the form I've been playing around with, trying to get it to ignore balloon. Ideally I could do something like (warning! - invalid regex below)
(?!b|c|B|C|)all
If anyone could point me in the right direction that would be great. The reason why I'm not looking for all surrounded by spaces is because I don't know if the reference I'm looking for is going to be:
.All
.all
("All")
(all)
and etc...

Have you tried: [aA][lL][lL]\b
any version of "all" or "ALL" anchored at a word/non-word boundary
Here is another reference..
Regular Expression to match specific string

The following regex: (?<!(b|c))all (with the IgnoreCase flag)
With the following input: ball all stall .all( "ALL"
Has the following matches: ball [all] st[all] .[all]( "[ALL]"

I think you are on the right track with lookarounds, but you want to use a character class with it. (More info: http://www.regular-expressions.info/charclass.html)
There are convenient shorthand character classes, like \w, to represent common classes. \w for example represents all alphanumeric characters and is shorthand for [A-Za-z0-9_].
\b represents a "word boundary," or in other words the beginning/end of the string and a boundary between a word character and a non-word character. It is zero-length and doesn't won't match any characters.
Here are some examples using a word boundary, positive lookaround, and negative lookaround respectively:
\b[aA][lL][lL]\b
(?<=[^\w])[aA][lL][lL](?=[^\w])
(?<!\w)[aA][lL][lL](?!\w)
Basically, these will find case-insensitive matches of "all" that are surrounded by non-alphanumeric characters. If you want to exclude certain surrounding characters, you can replace \w with your own character class (e.g. to exclude surrounding quote marks, use [A-Za-z0-9_"] instead of \w).

Noncapturing along with capturing match

I am trying to capture the subdomain from huge lists of domain names. For example I want to capture "funstuff" from "funstuff.mysite.com". I do not want to capture, ".mysite.com" in the match. These occurances are in a sea of text so I can not depend on them being at the start of a line. I know the subdomain will not include any special characters or numbers. So what I have is:
[a-z]{2,10}(?=\.mysite\.com)
The problem is this will work only if the subdomain is NOT preceded by a number or special character. For example, "asdfbasdasdfdfunstuff.mysite.com" will return "fdfunstuff" but "asdfasf23/funstuff.mysite.com" won't make a match.
I can not depend on there being a special character before the subdomain, like a "/" as in "http://funstuff.mysite.com" so that can not be used as part of the condition.
It is ok if the capture gets erroneous text before the subdomain, although 99% of the time it will be preceded with something other that a lowercase letter. I have tried,
(?<=[^a-z])[a-z]{2,10}(?=\.mysite\.com)
but for some reason this does not capture text is a situation like:
afb"asdfunstuff.mysite.com
Where the quotation mark prevents a match for [a-z]{2-20}. Basically what I would want to do in that case would be to capture asdfunstuff.mysite.com. How can this be accomplished?

So you've got two problems to solve: first, you want to match ".mysite.com" but not capture it; second, you want to grab up to 10 alphabetic characters in the "subdomain" position.
First problem can be solved by using a capturing group. The regex
([a-z]{2,10})\.mysite\.com
will capture somewhere between 2 and 10 characters, and the returned match object will expose that in one of its properties (depends on the language). C# returns a collection of Match objects, so it'll be the only item.
Second problem can be solved by using the word-boundary character \b. In .NET, this matches where an alphanumeric (i.e. \w) is next to a non-alphanumeric (\W). Other languages (e.g. ECMAScript / Javascript) work simliarly.
So, I suggest the following regex to solve your problem:
\b([a-z]{2,10})\.mysite\.com
Note that numbers are legal in subdomain names, too, so the following might be generally correct (though perhaps not in your specific case):
\b(\w{2,10})\.mysite\.com
where the "word character" \w is equivalent to [a-zA-Z_0-9] in .NET's ECMAScript-compliant mode. (Further reading.)

Counting special UTF-8 character

I'm finding a way to count special character that form by more than one character but found no solution online!
For e.g. I want to count the string "வாழைப்பழம". It actually consist of 6 tamil character but its 9 character in this case when we use the normal way to find the length. I am wondering is tamil the only kind of encoding that will cause this problem and if there is a solution to this. I'm currently trying to find a solution in C#.
Thank you in advance =)

Use StringInfo.LengthInTextElements:
var text = "வாழைப்பழம";
Console.WriteLine(text.Length); // 9
Console.WriteLine(new StringInfo(text).LengthInTextElements); // 6
The explanation for this behaviour can be found in the documentation of String.Length:
The Length property returns the number of Char objects in this instance, not the number of Unicode characters. The reason is that a Unicode character might be represented by more than one Char. Use the System.Globalization.StringInfo class to work with each Unicode character instead of each Char.

A minor nitpick: strings in .NET use UTF-16, not UTF-8
When you're talking about the length of a string, there are several different things you could mean:
Length in bytes.  This is the old C way of looking at things, usually.
Length in Unicode code points.  This gets you closer to the modern times and should be the way how string lengths are treated, except it isn't.
Length in UTF-8/UTF-16 code units.  This is the most common interpretation, deriving from 1. Certain characters take more than one code unit in those encodings which complicates things if you don't expect it.
Count of visible “characters” (graphemes). This is usually what people mean when they say characters or length of a string.
In your case your confusion stems from the difference between 4. and 3. 3. is what C# uses, 4. is what you expect. Complex scripts such as Tamil use ligatures and diacritics. Ligatures are contractions of two or more adjacent characters into a single glyph – in your case ழை is a ligature of ழ and ை – the latter of which changes the appearance of the former; வா is also such a ligature. Diacritics are ornaments around a letter, e.g. the accent in à or the dot above ப்.
The two cases I mentioned both result in a single grapheme (what you perceive as a single character), yet they both need two actual characters each. So you end up with three code points more in the string.
One thing to note: For your case the distinction between 2. and 3. is irrelevant, but generally you should keep it in mind.

Shall this Regex do what I expect from it, that is, matching against "A1:B10,C3,D4:E1000"?

I'm currently writing a library where I wish to allow the user to be able to specify spreadsheet cell(s) under four possible alternatives:
A single cell: "A1";
Multiple contiguous cells: "A1:B10"
Multiple separate cells: "A1,B6,I60,AA2"
A mix of 2 and 3: "B2:B12,C13:C18,D4,E11000"
Then, to validate whether the input respects these formats, I intended to use a regular expression to match against. I have consulted this article on Wikipedia:
Regular Expression (Wikipedia)
And I also found this related SO question:
regex matching alpha character followed by 4 alphanumerics.
Based on the information provided within the above-linked articles, I would try with this Regex:
Default Readonly Property Cells(ByVal cellsAddresses As String) As ReadOnlyDictionary(Of String, ICell)
Get
Dim validAddresses As Regex = New Regex("A-Za-z0-9:,A-Za-z0-9")
If (Not validAddresses.IsMatch(cellsAddresses)) then _
Throw New FormatException("cellsAddresses")
// Proceed with getting the cells from the Interop here...
End Get
End Property
Questions
1. Is my regular expression correct? If not, please help me understand what expression I could use.
2. What exception is more likely to be the more meaningful between a FormatException and an InvalidExpressionException? I hesitate here, since it is related to the format under which the property expect the cells to be input, aside, I'm using an (regular) expression to match against.
Thank you kindly for your help and support! =)

I would try this one:
[A-Za-z]+[0-9]+([:,][A-Za-z]+[0-9]+)*
Explanation:
Between [] is a possible group of characters for a single position
[A-Za-z] means characters (letters) from 'A' to 'Z' and from 'a' to 'z'
[0-9] means characters (digits) from 0 to 9
A "+" appended to a part of a regex means: repeat that one or more times
A "*" means: repeat the previous part zero or more times.
( ) can be used to define a group
So [A-Za-z]+[0-9]+ matches one or more letters followed by one or more digits for a single cell-address.
Then that same block is repeated zero or more times, with a ',' or ':' separating the addresses.

Assuming that the column for the spreadsheet is any 1- or 2-letter value and the row is any positive number, a more complex but tighter answer still would be:
^[A-Z]{1,2}[1-9]\d*(:[A-Z]{1,2}[1-9]\d*)?(,[A-Z]{1,2}[1-9]\d*(:[A-Z]{1,2}[1-9]\d*)?)*$
"[A-Z]{1,2}[1-9]\d*" is the expression for a single cell reference. If you replace "[A-Z]{1,2}[1-9]\d*" in the above with then the complex expression becomes
^<cell>(:<cell>)?(,<cell>(:<cell>*)?)*$
which more clearly shows that it is a cell or a range followed by one or more "cell or range" entries with commas in between.
The row and column indicators could be further refined to give a tighter still, yet more complex expression. I suspect that the above could be simplified with look-ahead or look-behind assertions, but I admit those are not (yet) my strong suit.

I'd go with this one, I think:
(([A-Z]+[1-9]\d*:)?[A-Z]+[1-9]\d*,)*([A-Z]+[1-9]\d*:)?[A-Z]+[1-9]\d*
This only allows capital letters as the prefix. If you want case insensitivity, use RegexOptions.IgnoreCase.
You could simplify this by replacing [A-Z]+[1-9]\d* with plain old [A-Z]\d+, but that will only allow a one-letter prefix, and it also allows stuff like A0 and B01. Up to you.
EDIT:
Having thought hard about DocMax's mention of lookarounds, and using Hans Kesting's answer as inspiration, it occurs to me that this should work:
^[A-Z]+\d+((,|(?<!:\w*):)[A-Z]+\d+)*$
Or if you want something really twisted:
^([A-Z]+\d+(,|$|(?<!:\w*):))*(?<!,|:)
As in the previous example, replace \d+ with [1-9]\d* if you want to prevent leading zeros.
The idea behind the ,|(?<!\w*:): is that if a group is delimited by a comma, you want to let it through; but if it's a colon, it's only allowed if the previous delimiter wasn't a colon. The (,|$|...) version is madness, but it allows you to do it all with only one [A-Z]+\d+ block.
However! Even though this is shorter, and I'll admit I feel a teeny bit clever about it, I pity the poor fellow who has to come along and maintain it six months from now. It's fun from a code-golf standpoint, but I think it's best for practical purposes to go with the earlier version, which is a lot easier to read.

i think your regex is incorrect, try (([A-Za-z0-9]*)[:,]?)*
Edit : to correct the bug pointed out by Baud : (([A-Za-z0-9]*)[:,]?)*([A-Za-z0-9]+)
and finally - best version : (([A-Za-z]+[0-9]+)[:,]?)*([A-Za-z]+[0-9]+)
// ah ok this wont work probably... but to answer 1. - no i dont think your regex is correct
( ) form a group
[ ] form a charclass (you can use A-Z a-d 0-9 etc or just single characters)
? means 1 or 0
* means 0 or any
id suggest reading http://www.regular-expressions.info/reference.html .
thats where i learned regexes some time ago ;)
and for building expressions i use Rad Software Regular Expression Designer

Let's build this step by step.
If you are following an Excel addressing format, to match a single-cell entry in your CSL, you would use the regular expression:
[A-Z]{1,2}[1-9]\d*
This matches the following in sequence:
Any character in A to Z once or twice
Any digit in 1 to 9
Any digit zero or more times
The digit expression will prevent inputting a cell address with leading zeros.
To build the expression that allows for a cell address pair, repeat the expression preceded by a colon as optional.
[A-Z]{1,2}[1-9]\d*(:[A-Z]{1,2}[1-9]\d*)?
Now allow for repeating the pattern preceded by a comma zero or more times and add start and end string delimiters.
^[A-Z]{1,2}[1-9]\d*(:[A-Z]{1,2}[1-9]\d*)?(,[A-Z]{1,2}[1-9]\d*(:[A-Z]{1,2}[1-9]\d*)?)*$
Kind of long and obnoxious, I admit, but after trying enough variants, I can't find a way of shortening it.
Hope this is helpful.