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Regex replace with bracket variable in C#

I am sure that has been asked before, but I cannot find the appropriate question(s).
Being new to C#'s Regex, I want to mimic what is possible e.g. with sed and awk where I would write s/_(20[0-9]{2})[.0-9]{1}/\1/g in order to find obtain a 4-digit year number after 2000 which is has an underscore as prefix and a number or a dot afterwards. The \1 refers to the value within brackets.
Example: Both files fx_201902.csv or fx_2019.csv should give me back myYear=2019. I was not successful with:
string myYear = Regex.Replace(Path.GetFileName(x), #"_20([0-9]{2})[.0-9]{1}", "\1")
How do I have to escape? Or is this kind of replacement not possible? If so, how would I do that?
Edit: My issue how to do the /1 in C#, in other words how to extract a regex-variable. Please forgive me my typos in the original post - I am trying the new SO app and I submitted earlier than intended.

I'd suggest more robust regex: _(20(?:0[1-9]|[1-9][0-9]))[\d.]
Explanation:
_ - match _ literally
(...) - first capturing group
20 - match 20 literally
(?:...) - non-capturing group
0[1-9]|[1-9][0-9] - alternation: match 0 and digit other than 0 OR match digit other then zero followed by any digits - this allows you to match ANY year after 2000
[\d.] - match dot or digit
And below is how you use capturing groups:
var regex = new Regex(#"_(20(?:0[1-9]|[1-9][0-9]))[\d.]");
regex.Match("fx_201902.csv").Groups[1].Value;
// "2019"
regex.Match("fx_20190.csv").Groups[1].Value;
// "2019"
regex.Match("fx_2019.csv").Groups[1].Value;
// "2019"

To extract the year using Regex.Replace, you need to capture only the year part of the string into a group and replace the entire string with just the capture group. That means you need to also match the characters before and after the year using (for example)
^.*_(20[0-9]{2})[.0-9].*$
That can then be replaced with $1 e.g.
Regex r = new Regex(#"^.*_(20[0-9]{2})[.0-9].*$");
string filename = "fx_201902.csv";
string myYear = r.Replace(filename, "$1");
Console.WriteLine(myYear);
filename = "fx_2019.csv";
myYear = r.Replace(filename, "$1");
Console.WriteLine(myYear);
Output:
2019
2019
If you want to exclude the year 2000 from your match, change the regex to
^.*_(20(?:0[1-9]|[1-9][0-9]))[.0-9].*$

You might use a capturing group for the first 4 digits and match what is before and after the 4 digits.
.*_(20[0-9]{2})[0-9]*\.\w+$
Explanation
.*_ Match the last underscore
(20[0-9]{2}) Match 20 and 2 digits
[0-9]*\. Match 0 or more occurrences of a digit followed by a dot
\w+$ Match 1 or or more word chars till the end of the string.
Regex demo | C# demo
In the replacement use:
$1
For example
string[] strings = {"fx_2019.csv", "fx_201902.csv"};
foreach (string s in strings)
{
string myYear = Regex.Replace(s, #".*_(20[0-9]{2})[0-9]*\.\w+$", "$1");
Console.WriteLine(myYear);
}
Output
2019
2019

Your second example does not contains the month's digits. If you still want to capture, make it optional:
Regex.Replace(Path.GetFileName(x), #"_20([1-9]{2})([.0-9]{2})?", "\1")
Note that I only added 3 characters to your query: (, ) and ?
If you want the returning value to be as expected: change the replacement to $1 from \1 as documented (with the correct parenthesis) and capture 2020, 2030, etc (still excluding 2000) with the usage of or operator and the combination of [0-9]{1} and [1-9]{1}:
Regex.Replace(Path.GetFileName(x), #"_(20(([1-9]{1})([0-9]{1})||([0-9]{1})([1-9]{1})))([.0-9]{2})?", "$1")
It worths mentioning that $3 and $4 matches the last and the 2nd last digit; and $2 matches with the last 2 digits (aka the combination of [0-9]{1} [1-9]{1} || [1-9]{1} [0-9]{1}).

How to match with Non-Ascii character using Regex in C#?

How to match 4 char then jump one char(which is unknown for me, so whatever may be such as some other chinese or special character occurance) after 4 char again jump one char(which is unknown for me, so whatever may be such as some other chinese or special character occurance) again 4 etc.,
My check string : 1234 4567 7891 0934
this is 16digit char, each 4char separated by space.
Main string:
"ACCOUNT NUMBER NAME STATEMENT DATE PAYMENT DUE DATE 1234 4567 7891 0934 Jane Doe 01/01/2009 02/26/09 CREDIT LIMIT CREDIT AVAILABLE NEW BALANCE MINIMUM PAYMENT DUE ."
above text(Main string) comes from PDF document. which was extracted by OCR Engine.
since Main string has my check string, but it's separated by some unknown char instead of space. I tried replace with # to space in Visual studio's immediate window. but that space of in-between Main string's check string was not replaced. thus, I could able to say It is Non-ascii character, but seems like a space.
I could be able get rid from this issue by below code:
string asAscii = Encoding.ASCII.GetString(
Encoding.Convert(
Encoding.UTF8,
Encoding.GetEncoding(
Encoding.ASCII.EncodingName,
new EncoderReplacementFallback(string.Empty),
new DecoderExceptionFallback()
),
Encoding.UTF8.GetBytes(inputString)
)
);
but,I would like to know Regex solution.
Even though non-ascii char occured, should be match with regex to check whether exists or not.

If you aren't sure whether the character between those 4 digits is a space or not, you can use a . character which matches any character and use this regex to match those group of 4 digits separated by a seemingly unknown character.
\d{4}.\d{4}.\d{4}.\d{4}
If you want to access those group of 4 digits, then you can put them in group and access them using all four grouping pattern from this regex,
(\d{4}).(\d{4}).(\d{4}).(\d{4})
Check this demo
Let me know if any of your query remains unresolved.

Regex pattern to match numbers in certain conditions

First time posting, please forgive the formatting. Not really a programmer, I work in C# with the Revit and AutoCAD APi's. Important to note, as the Revit API is a bit of mess, so the same code may produce different results in a different API. So I have three basic string patterns where I want to return certain numbers depending on what their prefix & suffix. They could be surrounded by other text than what I show, and the actual numbers and positions within the string may vary.
String 1: (12) #4x2'-0 # 6 EF
String 2: (12) #4 # 6 EF
String 3: STAGGER 2'-0, SPCG AT 6 AT 12 SLAB
The code I'm using:
if (LengthAsString.IsMatch(remarkdata) == true)
{
Regex remarklength = new Regex(#"isnertRegexPatternhere");
if (remarklength.IsMatch(remarkdata))
{
remarkdata = remarklength.Replace(remarkdata, "${0}\u0022");
}
}
remarkdata is the strings from above, and im adding inch marks " after each number match.
The patterns ive tested and their returns:
String 1 String 2 String 3
\d+(?!['-]|([(\d+)])) 0,6 4,6 0,6,12
(?<![#])\d+ 12,2,0,6 12,6 2,9,6,12
\d+(?= #)|(?<=# )\d+ 0,6 6 no matches
expected results: 0,6 6 0,6,12
so im close, but no cigar. Thoughts?
Double Edit: looking for the numbers that aren't preceded by #, nor between (). Ignore # and x, they may or may not be there.

You seem to be looking for
(?<!#)\d+(?!.*(?:['-]|[#x]\d))
See the regex demo
Details
(?<!#) - a negative lookbehind that fails the match if there is a # immediately to the left of the current location
\d+ - 1 or more digits (or [0-9]+ to only match ASCII digits)
(?!.*(?:['-]|[#x]\d)) - a negative lookahead that fails the match once there are any 0+ chars other than newline (.*) followed with ', -, or #/x followed with a digit immediately to the right of the current location.
Note that in case your strings always have balanced non-nested parentheses, and you may have (123) substrings after # or x1, you may also want to add [^()]*\) into the lookahead
(?<!#)\d+(?!.*(?:['-]|[#x]\d)|[^()]*\))
to avoid matching digits inside the parentheses.
See another .NET demo.

Using Regular Expression to find exact length match multiple times

I need a regular expression to find groups of exactly 8 numbers in a row. The closest I have gotten is:
[0-9]{8}
but it's not exactly what I need. If I had a number that was 9 long it will match the first 8 but I want it to ignore it if it's longer or shorter than 8.
Here are some examples
1234567890 <- no match, it's longer than 8
12345678 <- match: "12345678"
1234567809876543 <- match 1: "12345678", match 2: "09876543" (two groups of 8)
,,111-11-1234,12345678, <- match: "12345678"
To summarize, for every group of exactly 8 numbers make a match.
I'm working with some results of OCR (Optical Character Recognition) and I have to work with the shortcomings of the results so my input can be varied as in the examples above.
Here is some use case data: http://pastebin.com/uijF9K9n

You can use the following regex in .NET:
(?<=^|\D|(?:\d{8})+)\d{8}(?=$|\D|(?:\d{8})+)
See regex demo
It is based on variable-width lookbehind and a lookahead.
Regex breakdown:
(?<=^|\D|(?:\d{8})+) - only if at the string start (^), or preceded with not a digit (\D) or 1 or more sequences of 8 digits ((?:\d{8})+)...
\d{8} - match 8 digits that are followed by...
(?=$|\D|(?:\d{8})+) - either end of string ($) or not a digit (\D) or 1 or more sequences of 8 digits ((?:\d{8})+).
IMPORTANT:
If I got a downvote for the "extra" complexity compared with another answer, note our solutions are different: my regex matches 8-digit number in ID12345678, and the other one does not due to the word boundary.

You can also try this regex
(?:\b|\G)\d{8}(?=(?:\d{8})*\b)
(?:\b|\G) \b match a word boundary | or \G continue where last match attempt ended
\d{8} matches 8 digits [0-9] followed by a lookahead (?=... to check
(?:\d{8})*\b if followed by any amount of {8 digits} until another word boundary
It will match {8 digits} or out of a sequence of such if between two word boundaries.
See demo at regexstorm

\b[0-9]{8}\b this will give you what you want
For more details check this out
http://www.rexegg.com/regex-boundaries.html

Get a number with exactly x digits from string

im looking for a regex pattern, which matches a number with a length of exactly x (say x is 2-4) and nothing else.
Examples:
"foo.bar 123 456789", "foo.bar 456789 123", " 123", "foo.bar123 " has to match only "123"
So. Only digits, no spaces, letters or other stuff.
How do I have to do it?
EDIT: I want to use the Regex.Matches() function in c# to extract this 2-4 digit number and use it in additional code.

Any pattern followed by a {m,n} allows the pattern to occur m to n times. So in your case \d{m,n} for required values of m and n. If it has to be exactly an integer, use\d{m}
If you want to match 123 in x123y and not in 1234, use \d{3}(?=\D|$)(?<=(\D|^)\d{3})
It has a look ahead to ensure the character following the 3 digits is a non-digitornothing at all and a look behind to ensure that the character before the 3 digits is a non-digit or nothing at all.

You can achieve this with basic RegEx:
\b(\d\d\d)\b or \b(\d{3})\b - for matching a number with exactly 3 digits
If you want variable digits: \b(\d{2,4})\b (explained demo here)
If you want to capture matches next to words: \D(\d{2,4})\D (explained demo here)
\b is a word boundary (does not match anything, it's a zero-match character)
\d matches only digits
\D matches any character that is NOT a digit
() everything in round brackets will capture a match