Develop Reference

How to make regex only match with patterns that have exactly one letter before a =

I am trying to get the regex to match only when there is one letter from A-Z followed by a = like this A=, a=, B=, currently it is picking up any number of letters before the = like hem=, ac2=. Usually ^[a-zA-Z] works just fine but its not working for this case since I'm using named capture groups
String pattern = "FL2 (77) Flashing,77,a=1.875,A=90.0,b=3.625,B=95.0,c=1.375,C=175.0,d=2.5,hem=0.5,16GA-AL,";
var regex = new Regex("(?<label>[a-zA-Z]+)=(?<value>[^,]+)");
Other ways I've tried
var regex = new Regex("(?<label>^[a-zA-Z]+)=(?<value>[^,]+)");
var regex = new Regex("(?<label>[^a-zA-Z]+)=(?<value>[^,]+)");

If you want to match l= but not word=, you need a negative look-behind assertion.
new Regex("(?<![a-zA-Z])(?<label>[a-zA-Z])=(?<value>[^,]+)")

If the string pattern you have in your question is really the "haystack" in which you're looking for "needles", a really easy way to solve the problem would be to first split the string on ,, then use RegEx. Then you can use a simpler pattern ^(?<label>[a-zA-Z])=(?<value>.+)$ on each item in the list you get from splitting the string, and only keep the matches.

It's because you have a + after [a-zA-Z], which makes it match one or more characters in that character class. If you remove the +, it will only match one character before the =.
If you want it to only match in situations where there is exactly one alphabetical character before the equals sign, you will want to add to the beginning of the regex to make sure that the character before the letter you want to match is not a letter, like this:
(?<![a-zA-Z])(?<label>[a-zA-Z])=(?<value>[^,]+)
(notice though that this only matters in the case where you don't put a ^ before [a-zA-Z], in the case where you want matches that don't start at the beginning of a line)

Have you tried
var regex = new Regex("(?<label>^[a-zA-Z]?)=(?<value>[^,]+)");
I believe the "+" means 1 or more
"?" means 0 or 1
or exactly 1 should be {1} (at least in python, not sure about C#)
var regex = new Regex("(?<label>^[a-zA-Z]{1})=(?<value>[^,]+)");

Assuming that the label is separated by a comma (which seems to be the case based on your example and code) then you can use:
^|,(?<label>[A-Za-z])=(?<value>[^,]+)

I recommend Regex.Matches over capture groups here:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
namespace Rextester
{
public class Program
{
public static void Main(string[] args)
{
string content = "FL2 (77) Flashing,77,a=1.875,A=90.0,b=3.625,B=95.0,c=1.375,C=175.0,d=2.5,hem=0.5,16GA-AL,";
const string regexPattern = "(?<=[,| ])[a-zA-Z]=([0-9|.|-])+";
string singleMatch = new Regex(regexPattern).Match(content).ToString();
Console.WriteLine(singleMatch); // a=1.875
MatchCollection matchList = Regex.Matches(content, regexPattern);
var matches = matchList.Cast<Match>().Select(match => match.Value).ToList();
Console.WriteLine(string.Join(", ", matches)); // a=1.875, A=90.0, b=3.625, B=95.0, c=1.375, C=175.0, d=2.5
}
}
}

Using Regex to replace part of the entire string/expression

Regex are simple yet complex at times. Stuck to replace an expression having variables, assuming variable is of the following pattern:
\w+(\.\w+)*
I want to replace all the occurrences of my variable replacing dot (.) because i have to eventually tokenize the expression where tokenizer do not recognize variable having dots. So i thought to replace them with underscore before parsing. After tokenizing however i want to get the variable token with original value.
Expression:
(x1.y2.z3 + 9.99) + y2_z1 - x1.y2.z3
Three Variables:
x1.y2.z3
y2_z1
x1.y2.z3
Desired Output:
(x1_y2_z3 + 9.99) + y2_z1 - x1_y2_z3
Question 1: How to use Regex replace in this case?
Question 2: Is there any better way to address above mentioned problem because variable can have underscore so replacing dot with underscore is not a viable solution to get the original variable back in tokens?

This regex pattern seems to work: [a-zA-Z]+\d+\S+
To replace a dot found only in a match you use MatchEvaluator:
private static char charToReplaceWith = '_';
static void Main(string[] args)
{
string s = "(x1.y2.z3 + 9.99) + y2_z1 - x1.y2.z3";
Console.WriteLine(Regex.Replace(s, #"[a-zA-Z]+\d+\S+", new MatchEvaluator(ReplaceDotWithCharInMatch)));
Console.Read();
}
private static string ReplaceDotWithCharInMatch(Match m)
{
return m.Value.Replace('.', charToReplaceWith);
}
Which gives this output:
(x1_y2_z3 + 9.99) + y2_z1 - x1_y2_z3
I don't fully understand your second question and how to deal with tokenizing variables that already have underscores, but you should be able to choose a character to replace with (i.e., if (string.Contains('_')) is true then you choose a different character to replace with, but probably have to maintain a dictionary that says "I replaced all dots with underscores, and all underscores with ^, etc..).

Try this:
string input = "(x1.y2.z3 + 9.99) + y2_z1 - x1.y2.z3";
string output = Regex.Replace(input, "\\.(?<![a-z])", "_");
This will replace only periods which are followed by a letter (a-z).

Use Regex' negative lookahead by making a group that starts with (?!
A dot followed by something non-numeric would be as simple as this:
// matches any dot NOT followed by a character in the range 0-9
String output = Regex.Replace(input, "\\.(?![0-9])", "_");
This has the advantage that while the [0-9] is part of the expression, it is only checked as being behind the match, but is not actually part of the match.

Escape parts of string with a character in c# using regex

I have a string such as this:
/one/two/three-four/five six seven/eight/nine ten eleven-twelve
I need to first replace dashes with spaces, and then be able to escape any grouping of words that have a space between them with a "#" symbol, so the above string should be:
/one/two/#three four#/#five six seven#/eight/#nine ten eleven twelve#
I have the following extension method which works great for two words, but how can I make it work for any number of words.
public static string QueryEscape(this string str)
{
str = str.Replace("-", " ");
return Regex.Replace(str, #"(\w*) (\w*)", new MatchEvaluator(EscapeMatch));
}
private static string EscapeMatch(Match match)
{
return string.Format("#{0}#", match.Value);
}
So I guess I really need help with the proper regex that takes into account that
there could be any number of spaces
there may or may not be a trailing slash ("/")
takes into account that words are grouped between slashes, with the exception of #2 above.
Dashes are illegal and need to replaced with spaces
Thank you in advance for your support.

This should work for you:
public static string QueryEscape(this string str)
{
return Regex.Replace(str.Replace("-", " "), #"[^/]*(\s[^/]*)+", "#$&#");
}
Basically the idea is to match spans of text that isn't a slash that contains a (white-)space character in it. Then add the pound signs around the match.

Remove substring that starts with SOT and ends EOT, from string

I have a program that reads certain strings from memory. The strings contain, for the most part, recognizable characters. At random points in the strings however, "weird" characters appear. Characters I did not recognize. By going to a site that allows me to paste in Unicode characters to see what they are, I found that a selection of the "weird" characters were these:
\x{1} SOH, "start of heading", ctrl-a
\x{2} SOT, "start of text"
\x{3} EOT, "end of text"
\x{7} BEL, bell, ctrl-g
\x{13} dc3, device control three, ctrl-s
\x{11} dc1, device control one, ctrl-q
\x{14} dc4, device control four, ctrl-t
\x{1A} sub, substitute, ctrl-z
\x{6} ack, acknowledge, ctrl-f
I wanted to parse my strings to remove these characters. What I found out though, by looking at the strings, was that all the unwanted characters were always surrounded by the SOT and EOT, respectively.
Therefore, I am thinking that my question is: How can I remove, from a string, all occurrences of substrings that starts with SOT and ends with EOT?
Edit: Attempt at Solution
Using ideas from #RagingCain I made the following method:
private static string RemoveInvalidCharacters(string input)
{
while (true)
{
var start = input.IndexOf('\u0002');
var end = input.IndexOf('\u0003', start);
if (start == -1 || end == -1) break;
Console.WriteLine(#"Start: " + start + #". End: " + end);
var diff = end - start;
input = input.Remove(start, diff);
}
return input;
}
It does the trick, thanks again.

Regex would be your solution and should work fine. You would assign these characters to the Pattern and you can use the sub-method Match or even just Replace them with whitespace " ", or just cut them from the string all together by using "".
Regex.Replace: https://msdn.microsoft.com/en-us/library/xwewhkd1(v=vs.110).aspx
Regex.Match: https://msdn.microsoft.com/en-us/library/bk1x0726(v=vs.110).aspx
Regex example:
public static void Main()
{
string input = "This is text with far too much " +
"whitespace.";
string pattern = "\\s+";
string replacement = " ";
Regex rgx = new Regex(pattern);
string result = rgx.Replace(input, replacement);
Console.WriteLine("Original String: {0}", input);
Console.WriteLine("Replacement String: {0}", result);
}
I know the difficulty though of not being able to "see" them so you should assign them to Char variables by Unicode itself, add them to the pattern for replace.
Char Variables: https://msdn.microsoft.com/en-us/library/x9h8tsay.aspx
Unicode for Start of Text:
http://www.fileformat.info/info/unicode/char/0002/index.htm
Unicode for End of Text:
http://www.fileformat.info/info/unicode/char/0003/index.htm
To apply to your solution:
Does string contain SOT, EOT.
If true, remove entire string/sub-string/SOT or EOT.
It maybe easier to split original string into a string[], then go line by line... it's difficult to parse through your string without knowing what it looks like so hopefully I provided something that helps ^.^

Validate filename in c# through regex

I want to validate a filename with this format : LetterNumber_Enrollment_YYYYMMDD_HHMM.xml
string filename = "Try123_Enrollment_20130102_1200.xml";
Regex pattern = new Regex(#"[a-zA-z]_Enrollment_[0-9]{6}_[0-9]{4}\\.xml");
if (pattern.IsMatch(filename))
{
return isValid = true;
}
However, I can't make it to work.
Any thing that i missed here?

You are not matching digits at the beginning. Your pattern should be: ^[A-Za-z0-9]+_Enrollment_[0-9]{8}_[0-9]{4}\.xml$ to match given string.
Changes:
Your string starts with alphanumeric string before first _ symbol so you need to check both (letters and digits).
After Environment_ part you have digits with the length of 8 not 6.
No need of double \. You need to escape just dot (i.e. \.).
Demo app:
using System;
using System.Text.RegularExpressions;
class Test {
static void Main() {
string filename = "Try123_Enrollment_20130102_1200.xml";
Regex pattern = new Regex(#"^[A-Za-z0-9]+_Enrollment_[0-9]{8}_[0-9]{4}\.xml$");
if (pattern.IsMatch(filename))
{
Console.WriteLine("Matched");
}
}
}

Your Regex is nowhere near your actual string:
you only match a single letter at the start (and no digits) so Try123 doesn't match
you match 6 digits instead of 8 at the date part so 20130102 doesn't match
you have escaped your backslash near the end (\\.xml) but you've also used # on your string: with # you don't need to escape.
Try this instead:
#"[a-zA-Z]{3}\d{3}_Enrollment_[0-9]{8}_[0-9]{4}\.xml"
I've assumed you want only three letters and three numbers at the start; in fact you may want this:
#"[\w]*_Enrollment_[0-9]{8}_[0-9]{4}\.xml"

You can try the following, it matches letters and digits at the beginning and also ensures that the date is valid.
[A-Za-z0-9]+_Enrollment_(19|20)\d\d(0[1-9]|1[012])(0[1-9]|[12][0-9]|3[01])_[0-9]{4}\.xml
As an aside, to test your regular expressions try the free regular expression designer from Rad Software, I find that it helps me work out complex expressions beforehand.
http://www.radsoftware.com.au/regexdesigner/