Develop Reference

Regex Replace - Based on Char Input

Lets say we have string:
Hello
The user enters a char input "e"
What is the correct way of returning the string as the following using a regex method:
-e---
Code tried:
public static string updatedWord(char guess, string word)
{
string result = Regex.Replace(word, guess, "-");
console.writeline(result);
return result;
}

Assuming the input were e, you could build the following regex pattern:
[^e]
Then, do a global replacement on this pattern, which matches any single character which is not e, and replace it with a single dash.
string word = "Hello";
char guess = 'e';
string regex = "[^" + guess + "]";
string result = Regex.Replace(word, regex, "-");
Console.WriteLine(result);
This prints:
-e---
Note that to ensure that we handle regex metacharacters correctly, should they be allowed as inputs, we can wrap the regex pattern above in Regex.Escape:
Regex.Escape(regex)

This can be done without Regex, you need to "loop" all characters of the secret word and replace not yet guessed characters with -, regex will loop letters also, but c# methods are more comprehensible ;)
You need to keep collection of already guessed letters.
public class Guess
{
private readonly string _word;
private readonly HashSet<char> _guessed;
public Guess(string word)
{
_word = word;
_guessed = new HashSet<char>();
}
public string Try(char letter)
{
_guessed.Add(letter);
var maskedLetters = _word.Select(c => _guessed.Contains(c) ? c : '-').ToArray();
return new string(maskedLetters);
}
}
Usage
var game = new Guess("Hello");
var result = game.Try('e');
Console.WriteLine(result); // "-e---"

Find pattern to solve regex in one step

I have a problem to find the pattern that solves the problem in onestep.
The string looks like this:
Text1
Text1$Text2$Text3
Text1$Text2$Text3$Text4$Text5$Text6 etc.
What i want to get is: Take up to 4x Text. If there are more than "4xText" take only the last sign.
Example:
Text1$Text2$Text3$Text4$Text5$Text6 -> Text1$Text2$Text3$Text4&56
My current solution is:
First pattern:
^([^\$]*)\$?([^\$]*)\$?([^\$]*)\$?([^\$]*)\$?
After this i will do a substitution with the first pattern
New string: Text5$Text6
second pattern is:
([^\$])\b
result: 56
combine both and get the result:
Text1$Text2$Text3$Text4$56
For me it is not clear why i cant easily put the second pattern after the first pattern into one pattern. Is there something like an anchor that tells the engine to start the pattern from here like it would do if is would be the only pattern ?

You might use an alternation with a positive lookbehind and then concatenate the matches.
(?<=^(?:[^$]+\$){0,3})[^$]+\$?|[^$](?=\$|$)
Explanation
(?<= Positive lookbehind, assert what is on the left is
^(?:[^$]+\$){0,3} Match 0-3 times any char except $ followed by an optional $
) Close lookbehind
[^$]+\$? Match 1+ times any char except $, then match an optional $
| Or
[^$] Match any char except $
(?=\$|$) Positive lookahead, assert what is directly to the right is either $ or the end of the string
.NET regex demo | C# demo
Example
string pattern = #"(?<=^(?:[^$]*\$){0,3})[^$]*\$?|[^$](?=\$|$)";
string[] strings = {
"Text1",
"Text1$Text2$Text3",
"Text1$Text2$Text3$Text4$Text5$Text6"
};
Regex regex = new Regex(pattern);
foreach (String s in strings) {
Console.WriteLine(string.Join("", from Match match in regex.Matches(s) select match.Value));
}
Output
Text1
Text1$Text2$Text3
Text1$Text2$Text3$Text4$56

I strongly believe regular expression isn't the way to do that. Mostly because of the readability.
You may consider using simple algorithm like this one to reach your goal:
using System;
public class Program
{
public static void Main()
{
var input = "Text1$Text2$Text3$Text4$Text5$Text6";
var parts = input.Split('$');
var result = "";
for(var i=0; i<parts.Length; i++){
result += (i <= 4 ? parts[i] + "$" : parts[i].Substring(4));
}
Console.WriteLine(result);
}
}
There are also linq alternatives :
using System;
using System.Linq;
public class Program
{
public static void Main()
{
var input = "Text1$Text2$Text3$Text4$Text5$Text6";
var parts = input.Split('$');
var first4 = parts.Take(4);
var remainings = parts.Skip(4);
var result2 = string.Join("$", first4) + "$" + string.Join("", remainings.Select( r=>r.Substring(4)));
Console.WriteLine(result2);
}
}
It has to be adjusted to the actual needs but the idea is there

Try this code:
var texts = new string[] {"Text1", "Text1$Text2$Text3", "Text1$Text2$Text3$Text4$Text5$Text6" };
var parsed = texts
.Select(s => Regex.Replace(s,
#"(Text\d{1,3}(?:\$Text\d{1,3}){0,3})((?:\$Text\d{1,3})*)",
(match) => match.Groups[1].Value +"$"+ match.Groups[2].Value.Replace("Text", "").Replace("$", "")
)).ToArray();
// parsed is now: string[3] { "Text1$", "Text1$Text2$Text3$", "Text1$Text2$Text3$Text4$56" }
Explanation:
solution uses regex pattern: (Text\d{1,3}(?:\$Text\d{1,3}){0,3})((?:\$Text\d{1,3})*)
(...) - first capturing group
(?:...) - non-capturing group
Text\d{1,3}(?:\$Text\d{1,3} - match Text literally, then match \d{1,3}, which is 1 up to three digits, \$ matches $ literally
Rest is just repetition of it. Basically, first group captures first four pieces, second group captures the rest, if any.
We also use MatchEvaluator here which is delegate type defined as:
public delegate string MatchEvaluator(Match match);
We define such method:
(match) => match.Groups[1].Value +"$"+ match.Groups[2].Value.Replace("Text", "").Replace("$", "")
We use it to evaluate match, so takee first capturing group and concatenate with second, removing unnecessary text.

It's not clear to me whether your goal can be achieved using exclusively regex. If nothing else, the fact that you want to introduce a new character '&' into the output adds to the challenge, since just plain matching would never be able to accomplish that. Possibly using the Replace() method? I'm not sure that would work though...using only a replacement pattern and not a MatchEvaluator, I don't see a way to recognize but still exclude the "$Text" portion from the fifth instance and later.
But, if you are willing to mix regex with a small amount of post-processing, you can definitely do it:
static readonly Regex regex1 = new Regex(#"(Text\d(?:\$Text\d){0,3})(?:\$Text(\d))*", RegexOptions.Compiled);
static void Main(string[] args)
{
for (int i = 1; i <= 6; i++)
{
string text = string.Join("$", Enumerable.Range(1, i).Select(j => $"Text{j}"));
WriteLine(KeepFour(text));
}
}
private static string KeepFour(string text)
{
Match match = regex1.Match(text);
if (!match.Success)
{
return "[NO MATCH]";
}
StringBuilder result = new StringBuilder();
result.Append(match.Groups[1].Value);
if (match.Groups[2].Captures.Count > 0)
{
result.Append("&");
// Have to iterate (join), because we don't want the whole match,
// just the captured text.
result.Append(JoinCaptures(match.Groups[2]));
}
return result.ToString();
}
private static string JoinCaptures(Group group)
{
return string.Join("", group.Captures.Cast<Capture>().Select(c => c.Value));
}
The above breaks your requirement into three different capture groups in a regex. Then it extracts the captured text, composing the result based on the results.

Document filtering by regex

I'm trying to find best solution to verify input document. I need to check every line of the document. Basically in each line can exist invalid character or characters. The result of searching (validating) is: 'get me the index of line with invalid char and index of each invalid character in this line'.
I know how to do in standard way (open file -> read all lines -> check characters one by one), but this method isn't best optimized way. Instead of this, the best solution will be to use "MatchCollection" (in my opinion).
But how to do this correctly in C# ?
Link:
http://www.dotnetperls.com/regex-matches
Example:
"Some Înput text here,\n Îs another lÎne of thÎs text."
In first line [0] found invalid character on [6] index, in line [1]
found invalid characters on [0, 12, 21] index.
using System;
using System.Text.RegularExpressions;
namespace RegularExpresion
{
class Program
{
private static Regex regex = null;
static void Main(string[] args)
{
string input_text = "Some Înput text here, Îs another lÎne of thÎs text.";
string line_pattern = "\n";
string invalid_character = "Î";
regex = new Regex(line_pattern);
/// Check is multiple or single line document
if (IsMultipleLine(input_text))
{
/// ---> How to do this correctly for each line ? <---
}
else
{
Console.WriteLine("Is a single line file");
regex = new Regex(invalid_character);
MatchCollection mc = regex.Matches(input_text);
Console.WriteLine($"How many matches: {mc.Count}");
foreach (Match match in mc)
Console.WriteLine($"Index: {match.Index}");
}
Console.ReadKey();
}
public static bool IsMultipleLine(string input) => regex.IsMatch(input);
}
}
Output:
Is a single line file
How many matches: 4
Index: 5
Index: 22
Index: 34
Index: 43

Link:
http://www.dotnetperls.com/regexoptions-multiline
SOLUTION
using System;
using System.Text.RegularExpressions;
namespace RegularExpresion
{
class Program
{
private static Regex regex = null;
static void Main(string[] args)
{
string input_text = #"Some Înput text here,
Îs another lÎne of thÎs text.";
string line_pattern = "\n";
string invalid_character = "Î";
regex = new Regex(line_pattern);
/// Check is multiple or single line document
if (IsMultipleLine(input_text))
{
Console.WriteLine("Is a multiple line file");
MatchCollection matches = Regex.Matches(input_text, "^(.+)$", RegexOptions.Multiline);
int line = 0;
foreach (Match match in matches)
{
foreach (Capture capture in match.Captures)
{
line++;
Console.WriteLine($"Line: {line}");
RegexpLine(capture.Value, invalid_character);
}
}
}
else
{
Console.WriteLine("Is a single line file");
RegexpLine(input_text, invalid_character);
}
Pause();
}
public static bool IsMultipleLine(string input) => regex.IsMatch(input);
public static void RegexpLine(string line, string characters)
{
regex = new Regex(characters);
MatchCollection mc = regex.Matches(line);
Console.WriteLine($"How many matches: {mc.Count}");
foreach (Match match in mc)
Console.WriteLine($"Index: {match.Index}");
}
public static ConsoleKeyInfo Pause(string message = "please press ANY key to continue...")
{
Console.WriteLine(message);
return Console.ReadKey();
}
}
}
Thx guys for help, basically will be nice if someone smarter then me, check this code in terms of performance.
Regards,
Nerus.

My approach would be split the string into array of string, each contains a line. If the length of the array is just 1, that means you have only 1 line. Then from there you use the Regex to match each line to find the invalid character that you are looking for.
string input_text = "Some Înput text here,\nÎs another lÎne of thÎs text.";
string line_pattern = "\n";
// split the string into string arrays
string[] input_texts = input_text.Split(new string[] { line_pattern }, StringSplitOptions.RemoveEmptyEntries);
string invalid_character = "Î";
if (input_texts != null && input_texts.Length > 0)
{
if (input_texts.Length == 1)
{
Console.WriteLine("Is a single line file");
}
// loop every line
foreach (string oneline in input_texts)
{
Regex regex = new Regex(invalid_character);
MatchCollection mc = regex.Matches(oneline);
Console.WriteLine("How many matches: {0}", mc.Count);
foreach (Match match in mc)
{
Console.WriteLine("Index: {0}", match.Index);
}
}
}
--- EDIT ---
Things to consider:
If you get your input from a file, I would recommend you to read line by line, not the whole text.
Usually, when you search for invalid character, you don't specify it. Instead you look for a pattern. For ex: Not a char from a-z, A-Z, 0-9. Then your regex is going to be a little bit different.

Replace regular expression with regular expression

Consider two regular expressions:
var regex_A = "Main\.(.+)\.Value";
var regex_B = "M_(.+)_Sp";
I want to be able to replace a string using regex_A as input, and regex_B as the replacement string. But also the other way around. And without supplying additional information like a format string per regex.
Specifically I want to create a replaced_B string from an input_A string. So:
var input_A = "Main.Rotating.Value";
var replaced_B = input_A.RegEx_Awesome_Replace(regex_A, regex_B);
Assert.AreEqual("M_Rotating_Sp", replaced_B);
And this should also work in reverse (thats the reason i can't use a simple string.format for regex_B). Because I don't want to supply a format string for every regular expression (i'm lazy).
var input_B = "M_Skew_Sp";
var replaced_A = input_B.RegEx_Awesome_Replace(regex_B, regex_A);
Assert.AreEqual("Main.Skew.Value", replaced_A);
I have no clue if this exists, or how to call it. Google search finds me all kinds of other regex replaces... not this one.
Update:
So basically I need a way to convert a regular expression to a format string.
var regex_A_format = Regex2Format(regex_A);
Assert.AreEqual("Main.$1.Value", regex_A_format);
and
var regex_B_format = Regex2Format(regex_B);
Assert.AreEqual("M_$1_Sp", regex_B_format);
So what should the RegEx_Awesome_Replace and/or Regex2Format function look like?
Update 2:
I guess the RegEx_Awesome_Replace should look something like (using some code from answers below):
public static class StringExtenstions
{
public static string RegExAwesomeReplace(this string inputString,string searchPattern,string replacePattern)
{
return Regex.Replace(inputString, searchPattern, Regex2Format(replacePattern));
}
}
Which would leave the Regex2Format as an open question.

There is no defined way for one regex to refer to a match found in another regex. Regexes are not format strings.
What you can do is to use Tuples of a format string together with its regex. e.g.
var a = new Tuple<Regex,string>(new Regex(#"(?<=Main\.).+(?=\.Value)"), #"Main.{0}.Value")
var b = new Tuple<Regex,string>(new Regex(#"(?<=M_).+(?=_Sp)"), #"M_{0}_Sp")`
Then you can pass these objects to a common replacement method in any order, like this:
private string RegEx_Awesome_Replace(string input, Tuple<Regex,string> toFind, Tuple<Regex,string> replaceWith)
{
return string.Format(replaceWith.Item2, toFind.Item1.Match(input).Value);
}
You will notice that I have used zero-width positive lookahead assertion and zero-width positive lookbehind assertions in my regexes, to ensure that Value contains exactly the text that I want to replace.
You may also want to add error handling, for cases where the match can not be found. Maybe read about Regex.Match

Since you have already reduced your problem to where you need to change a Regex into a string format (implementing Regex2Format) I will focus my answer just on that part. Note that my answer is incomplete because it doesn't address the full breadth of parsing regex capturing groups, however it works for simple cases.
First thing needed is a Regex that will match Regex capture groups. There is a negative lookbehind to not match escaped bracket symbols. There are other cases that break this regex. E.g. a non-capturing group, wildcard symbols, things between square braces.
private static readonly Regex CaptureGroupMatcher = new Regex(#"(?<!\\)\([^\)]+\)");
The implementation of Regex2Format here basically writes everything outside of capture groups into the output string, and replaces the capture group value by {x}.
static string Regex2Format(string pattern)
{
var targetBuilder = new StringBuilder();
int previousEndIndex = 0;
int formatIndex = 0;
foreach (Match match in CaptureGroupMatcher.Matches(pattern))
{
var group = match.Groups[0];
int endIndex = group.Index;
AppendPart(pattern, previousEndIndex, endIndex, targetBuilder);
targetBuilder.Append('{');
targetBuilder.Append(formatIndex++);
targetBuilder.Append('}');
previousEndIndex = group.Index + group.Length;
}
AppendPart(pattern, previousEndIndex, pattern.Length, targetBuilder);
return targetBuilder.ToString();
}
This helper function writes pattern string values into the output, it currently writes everything except \ characters used to escape something.
static void AppendPart(string pattern, int previousEndIndex, int endIndex, StringBuilder targetBuilder)
{
for (int i = previousEndIndex; i < endIndex; i++)
{
char c = pattern[i];
if (c == '\\' && i < pattern.Length - 1 && pattern[i + 1] != '\\')
{
//backslash not followed by another backslash - it's an escape char
}
else
{
targetBuilder.Append(c);
}
}
}
Test cases
static void Test()
{
var cases = new Dictionary<string, string>
{
{ #"Main\.(.+)\.Value", #"Main.{0}.Value" },
{ #"M_(.+)_Sp(.*)", "M_{0}_Sp{1}" },
{ #"M_\(.+)_Sp", #"M_(.+)_Sp" },
};
foreach (var kvp in cases)
{
if (PatternToStringFormat(kvp.Key) != kvp.Value)
{
Console.WriteLine("Test failed for {0} - expected {1} but got {2}", kvp.Key, kvp.Value, PatternToStringFormat(kvp.Key));
}
}
}
To wrap up, here is the usage:
private static string AwesomeRegexReplace(string input, string sourcePattern, string targetPattern)
{
var targetFormat = PatternToStringFormat(targetPattern);
return Regex.Replace(input, sourcePattern, match =>
{
var args = match.Groups.OfType<Group>().Skip(1).Select(g => g.Value).ToArray<object>();
return string.Format(targetFormat, args);
});
}

Something like this might work
var replaced_B = Regex.Replace(input_A, #"Main\.(.+)\.Value", #"M_$1_Sp");

Are you looking for something like this?
public static class StringExtenstions
{
public static string RegExAwesomeReplace(this string inputString,string searchPattern,string replacePattern)
{
Match searchMatch = Regex.Match(inputString,searchPattern);
Match replaceMatch = Regex.Match(inputString, replacePattern);
if (!searchMatch.Success || !replaceMatch.Success)
{
return inputString;
}
return inputString.Replace(searchMatch.Value, replaceMatch.Value);
}
}
The string extension method returns the string with replaced value for search pattern and replace pattern.
This is how you call:
input_A.RegEx_Awesome_Replace(regex_A, regex_B);

Regex to skip certain characters

I want to write a Regex which would skip characters like < & >. Reason
Now, to represent this I came across this [^<>] and tried using it in an console application, but it does not work.
[^<>]
Debuggex Demo
string value = "shubh<";
string regEx = "[^<>]";
Regex rx = new Regex(regEx);
if (rx.IsMatch(value))
{
Console.WriteLine("Pass");
}
else { Console.WriteLine("Fail"); }
Console.ReadLine();
The string 'shubh<' should get failed, but I am not sure why it passes the match. Am I doing something rubbish?

From Regex.IsMatch Method (String):
Indicates whether the regular expression specified in the Regex constructor finds a match in a specified input string.
[^<>] is found in shubh< (the s, the h, etc.).
You need to use the ^ and $ anchors:
Regex rx = new Regex("^[^<>]*$");
if (rx.IsMatch(value)) {
Console.WriteLine("Pass");
} else {
Console.WriteLine("Fail");
}
Another solution is to check if < or > is contained:
Regex rx = new Regex("[<>]");
if (rx.IsMatch(value)) {
Console.WriteLine("Fail");
} else {
Console.WriteLine("Pass");
}