Develop Reference

Compare if input string is in correct format

Check if input string entered by user is in format like IIPIII, where I is integer number, any one digit number can be used on place of I and P is a character.
Example if input is 32P125 it is valid string else N23P33 is invalid.
I tried using string.Length or string.IndexOf("P") but how to validate other integer values?

I'm sure someone can offer a more succinct answer but pattern matching is the way to go.
using System.Text.RegularExpressions;
string test = "32P125";
// 2 integers followed by any upper cased letter, followed by 3 integers.
Regex regex = new Regex(#"\d{2}[A-Z]\d{3}", RegexOptions.ECMAScript);
Match match = regex.Match(test);
if (match.Success)
{
//// Valid string
}
else
{
//// Invalid string
}

Considering that 'P' has to matched literally -
using System;
using System.Text.RegularExpressions;
public class Program
{
public static void Main()
{
string st1 = "32P125";
string st2 = "N23P33";
Regex rg = new Regex(#"\d{2}P\d{3}");
// If 'P' is not to be matched literally, reeplace above line with below one
// Regex rg = new Regex(#"\d{2}[A-Za-z]\d{3}");
Console.WriteLine(rg.IsMatch(st1));
Console.WriteLine(rg.IsMatch(st2));
}
}
OUTPUT
True
False

It can be encapsulated in one simple if:
string testString = "12P123";
if(
// check if third number is letter
Char.IsLetter(testString[2]) &&
// if above succeeds, code proceeds to second condition (short-circuiting)
// remove third character, after that it should be a valid number (only digits)
int.TryParse(testString.Remove(2, 1), out int i)
) {...}

I would encourage the usage of MaskedTextProvided over Regex.
Not only is this looking cleaner but it's also less error prone.
Sample code would look like the following:
string Num = "12P123";
MaskedTextProvider prov = new MaskedTextProvider("##P###");
prov.Set(Num);
var isValid = prov.MaskFull;
if(isValid){
string result = prov.ToDisplayString();
Console.WriteLine(result);
}

Use simple Regular expression for this kind of stuff.

Using regex, match a string and then pull integer value out of match

I am trying to:
match the given input if it has the [ process id N ] where N can be any integer value (actually positive value).
return the N integer value from the match.
The following seems to work in a two phase call but is there (should be) a way to both match the string and pull the integer out in one call to the Regex?
using System;
using System.Text.RegularExpressions;
namespace ConsoleApplication25
{
class Program
{
static void Main()
{
string instanceName = "message read rate [ process id 1776 ]";
Regex expression = new Regex(#".*process id (\d).*");
var matches = expression.Match(instanceName);
string processId = Regex.Match(matches.Value, #"\d+").Value;
Console.WriteLine(processId);
}
}
}

If you care about performance and your input string is large, you will want to drop the .* that you have used in your regex at the beginning and end, because they really serve no purpose whatsoever.
Secondly, you certainly can use (\d+) in your first regex to get all the numbers within the process ID instead of a single number with (\d) (as several have already mentioned). You can then access it through matches.Groups[1].Value.
Last, it is safer if you use if (matches.Success), just so you don't get errors when there is no match:
using System;
using System.Text.RegularExpressions;
namespace ConsoleApplication25
{
class Program
{
static void Main()
{
string instanceName = "message read rate [ process id 1776 ]";
Regex expression = new Regex(#"process id (\d+)");
var matches = expression.Match(instanceName);
if (matches.Success)
{
Console.WriteLine("Process ID: " + matches.Groups[1].Value);
}
else
{
Console.WriteLine("No match found");
}
}
}
}
As to why removing .* makes the regex less efficient, you might want to read on greedy quantifiers and backtracking. In simple terms, .* will match everything till the end of the string (except newlines, unless the DOTALL flag is active, where it'll be able to match even more and reduce efficiency even more) and then will go back one character at a time to get the other matches in the regex. The more characters in the string, the slower it becomes since there's more to backtrack.
The .Match function doesn't need to match the whole string; it will find a match anywhere in the string.

var match = expression.Match(instanceName);
var processId = Int32.Parse(match.Groups[1].Value);

Do this:
string instanceName = "message read rate [ process id 1776 ]";
var s = Regex.Match(instanceName, #".*process id (\d+).*");
Console.WriteLine(s.Groups[1]);
Instead of this:
string instanceName = "message read rate [ process id 1776 ]";
Regex expression = new Regex(#".*process id (\d).*");
var matches = expression.Match(instanceName);
string processId = Regex.Match(matches.Value, #"\d+").Value;
Console.WriteLine(processId);

While the answers about using Groups are correct, I prefer to use Named Groups. For your example, it may be overkill, but when you start to use more complex regex's, it is easier to keep track of what the various groups are:
string instanceName = "message read rate [ process id 1776 ]";
string expression = #".*process id (?<PROCESS_ID>\d+).*";
Match match = Regex.Match(instanceName, expression);
if (match.Success)
{
string processId = match.Groups["PROCESS_ID"].Value.Trim();
Console.WriteLine("Process ID is {0}", processId);
}
else
{
Console.WriteLine("Could not find process id");
}

Change your regex to this
(?<=process id )\d+
This will match the id number only

No. Regex is made for searching texts and parsing numerics is meant to be done by yourself.
To the downvoter: the question was not how to extract a number using regex, but how to extract the number as an integer straight by regex engine, which is unrealizable.

Extracting Numbers from String RegEx

I am really struggling with Regular Expressions and can't seem to extract the number from this string
"id":143331539043251,
I've tried with this ... but I'm getting compilation errors
var regex = new Regex(#""id:"\d+,");
Note that the full string contains other numbers I don't want. I want numbers between id: and the ending ,

Try this code:
var match = Regex.Match(input, #"\""id\"":(?<num>\d+)");
var yourNumber = match.Groups["num"].Value;
Then use extracted number yourNumber as a string or parse it to number type.

If all you need is the digits, just match on that:
[0-9]+
Note that I am not using \d as that would match on any digit (such as Arabic numerals) in the .NET regex engine.
Update, following comments on the question and on this answer - the following regex will match the pattern and place the matched numbers in a capturing group:
#"""id"":([0-9]+),"
Used as:
Regex.Match(#"""id"":143331539043251,", #"""id"":([0-9]+),").Groups[1].Value
Which returns 143331539043251.

If you are open to using LINQ try the following (c#):
string stringVariable = "123cccccbb---556876---==";
var f = (from a in stringVariable.ToCharArray() where Char.IsDigit(a) == true select a);
var number = String.Join("", f);

Using regex to capture a numeric value within a string in C#

I have a string of characters which has 0 or more occurrences of ABC = dddd within it. The dddd stands for an integer value, not necessarily four digits.
What I'd like to do is capture the integer values that occur within this pattern. I know how to perform matches with regexes but I'm new to capturing. It's not necessary to capture all the ABC integer values in one call—it's fine to loop over the string.
If this is too involved I'll just write a tiny parser, but I'd like to use regex if it's reasonably elegant. Expertise greatly appreciated.

First we need to start with a regex that matches the pattern we are looking for. This will match the example you have given (assuming ABC is alphanumeric): \w+\s*=\s*\d+
Next we need to define what we want to capture in a match by defining capture groups. .Net includes support for named capture groups, which I absolutely adore. We specify a group with (?<name for capture>expression), turning our regex into: (?<key>\w+)\s*=\s*(?<value>\d+). This gives us two captures, key and value.
Using this, we can iterate over all matches in your text:
Regex pattern = new Regex(#"(?<key>\w+)\s*=\s*(?<value>\d+)");
string body = "This is your text here. value = 1234";
foreach (Match match in pattern.Matches(body))
{
Console.WriteLine("Found key {0} with value {1}",
match.Groups.Item["key"].Value,
match.Groups.Item["value"].Value
);
}

You can use something like this:
MatchCollection allMatchResults = null;
try {
// This matches a literal '=' and then any number of digits following
Regex regexObj = new Regex(#"=(\d+)");
allMatchResults = regexObj.Matches(subjectString);
if (allMatchResults.Count > 0) {
// Access individual matches using allMatchResults.Item[]
} else {
// Match attempt failed
}
} catch (ArgumentException ex) {
// Syntax error in the regular expression
}
Based on your coment, perhaps this is more what you're after:
try {
Regex regexObj = new Regex(#"=(\d+)");
Match matchResults = regexObj.Match(subjectString);
while (matchResults.Success) {
for (int i = 1; i < matchResults.Groups.Count; i++) {
Group groupObj = matchResults.Groups[i];
if (groupObj.Success) {
// matched text: groupObj.Value
// match start: groupObj.Index
// match length: groupObj.Length
}
}
matchResults = matchResults.NextMatch();
}
} catch (ArgumentException ex) {
// Syntax error in the regular expression
}

Regular Expression to match numbers inside parenthesis inside square brackets with optional text

Firstly, I'm in C# here so that's the flavor of RegEx I'm dealing with. And here are thing things I need to be able to match:
[(1)]
or
[(34) Some Text - Some Other Text]
So basically I need to know if what is between the parentheses is numeric and ignore everything between the close parenthesis and close square bracket. Any RegEx gurus care to help?

This should work:
\[\(\d+\).*?\]
And if you need to catch the number, simply wrap \d+ in parentheses:
\[\((\d+)\).*?\]

Do you have to match the []? Can you do just ...
\((\d+)\)
(The numbers themselves will be in the groups).
For example ...
var mg = Regex.Match( "[(34) Some Text - Some Other Text]", #"\((\d+)\)");
if (mg.Success)
{
var num = mg.Groups[1].Value; // num == 34
}
else
{
// No match
}

Regex seems like overkill in this situation. Here is the solution I ended up using.
var src = test.IndexOf('(') + 1;
var dst = test.IndexOf(')') - 1;
var result = test.SubString(src, dst-src);

Something like:
\[\(\d+\)[^\]]*\]
Possibly with some more escaping required?

How about "^\[\((d+)\)" (perl style, not familiar with C#). You can safely ignore the rest of the line, I think.

Depending on what you're trying to accomplish...
List<Boolean> rslt;
String searchIn;
Regex regxObj;
MatchCollection mtchObj;
Int32 mtchGrp;
searchIn = #"[(34) Some Text - Some Other Text] [(1)]";
regxObj = new Regex(#"\[\(([^\)]+)\)[^\]]*\]");
mtchObj = regxObj.Matches(searchIn);
if (mtchObj.Count > 0)
rslt = new List<bool>(mtchObj.Count);
else
rslt = new List<bool>();
foreach (Match crntMtch in mtchObj)
{
if (Int32.TryParse(crntMtch.Value, out mtchGrp))
{
rslt.Add(true);
}
}

How's this? Assuming you only need to determine if the string is a match, and need not extract the numeric value...
string test = "[(34) Some Text - Some Other Text]";
Regex regex = new Regex( "\\[\\(\\d+\\).*\\]" );
Match match = regex.Match( test );
Console.WriteLine( "{0}\t{1}", test, match.Success );