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search string for everything before a set of characters in C#

I'm looking for a way to search a string for everything before a set of characters in C#. For Example, if this is my string value:
This is is a test.... 12345
I want build a new string with all of the characters before "12345".
So my new string would equal "This is is a test.... "
Is there a way to do this?
I've found Regex examples where you can focus on one character but not a sequence of characters.

You don't need to use a Regex:
public string GetBitBefore(string text, string end)
{
var index = text.IndexOf(end);
if (index == -1) return text;
return text.Substring(0, index);
}

You can use a lazy quantifier to match anything, followed by a lookahead:
var match = Regex.Match("This is is a test.... 12345", #".*?(?=\d{5})");
where:
.*? lazily matches everything (up to the lookahead)
(?=…) is a positive lookahead: the pattern must be matched, but is not included in the result
\d{5} matches exactly five digits. I'm assuming this is your lookahead; you can replace it

You can do so with help of regex lookahead.
.*(?=12345)
Example:
var data = "This is is a test.... 12345";
var rxStr = ".*(?=12345)";
var rx = new System.Text.RegularExpressions.Regex (rxStr,
System.Text.RegularExpressions.RegexOptions.IgnoreCase);
var match = rx.Match(data);
if (match.Success) {
Console.WriteLine (match.Value);
}
Above code snippet will print every thing upto 12345:
This is is a test....
For more detail about see regex positive lookahead

This should get you started:
var reg = new Regex("^(.+)12345$");
var match = reg.Match("This is is a test.... 12345");
var group = match.Groups[1]; // This is is a test....
Of course you'd want to do some additional validation, but this is the basic idea.

^ means start of string
$ means end of string
The asterisk tells the engine to attempt to match the preceding token zero or more times. The plus tells the engine to attempt to match the preceding token once or more
{min,max} indicate the minimum/maximum number of matches.
\d matches a single character that is a digit, \w matches a "word character" (alphanumeric characters plus underscore), and \s matches a whitespace character (includes tabs and line breaks).
[^a] means not so exclude a
The dot matches a single character, except line break characters
In your case there many way to accomplish the task.
Eg excluding digit: ^[^\d]*
If you know the set of characters and they are not only digit, don't use regex but IndexOf(). If you know the separator between first and second part as "..." you can use Split()

Take a look at this snippet:
class Program
{
static void Main(string[] args)
{
string input = "This is is a test.... 12345";
// Here we call Regex.Match.
MatchCollection matches = Regex.Matches(input, #"(?<MySentence>(\w+\s*)*)(?<MyNumberPart>\d*)");
foreach (Match item in matches)
{
Console.WriteLine(item.Groups["MySentence"]);
Console.WriteLine("******");
Console.WriteLine(item.Groups["MyNumberPart"]);
}
Console.ReadKey();
}
}

You could just split, not as optimal as the indexOf solution
string value = "oiasjdoiasj12345";
string end = "12345";
string result = value.Split(new string[] { end }, StringSplitOptions.None)[0] //Take first part of the result, not the quickest but fairly simple

Write a regular expression in order to search a substring C#

I have a string which I want to examine and search for a substring within it. If the substring is found, I want to do something on the original string.
The string looks like this:
"\r\radmin#Modem -- *<456> \radmin#Modem -- *<456> "
Goal: Search the substring pattern " -- *<456> " if it exists in the string, and return success or fail (the digits number is between 1 to infinite: 1, 5, 36, 76, 478, 975 etc.).
What is the regular expression pattern which I need?

Use this:
var myRegex = new Regex("(?<=<)[0-9]+(?=>)");
string resultString = myRegex.Match(yourString).Value;
Console.WriteLine(resultString);
// matches 456
See the match in the Regex Demo.
Explanation
The lookbehind (?<=<) asserts that what precedes is <
[0-9]+ matches one or more digits
The lookahead (?=>) asserts that what follows is >

You can use this following piece of code to check if your pattern exist :
string yourInput = "\r\radmin#Modem -- *<456> \radmin#Modem -- *<456> " ;
string pattern = #"<(\d+)>";
boolean success = Regex.Match(yourInput , pattern, RegexOptions.IgnoreCase).Success ;
success will be true if a number is found.

With this pattern you can match the string: "--\s\*\<\d{3}\>"
Note: If the number of digits can change, use this: "--\s\*\<\d{MIN,MAX}\>" where MIN and MAX are the number of digits that can appear in your string (within the part we are interested in matching).
using System;
using System.Text.RegularExpressions;
class Example
{
static void Main()
{
string text = "One car red car blue car";
// This regex will match the pattern you're looking for
// Since youre new to regexes :) I'll explain it a little:
// "--" matches "--" literally, "\s" matches the space in between but only once.
// "\*" matches the "*" and "\<" and "\>" match "<" and ">" respectively
// "\d" matches a digit 0-9 and "{3}" indicates that there are three digits
string pat = #"--\s\*\<\d{3}\>";
// Instantiate the regular expression object.
Regex r = new Regex(pat, RegexOptions.IgnoreCase);
// Match the regular expression pattern against a text string.
Match m = r.Match(text);
while (m.Success)
{
// Do something ...
// Find next match
m = m.NextMatch();
}
}
}
This will allow you to make any changes on a per match basis. So every time you match the regex you can do something to your string and then look if there is another match and so on...

Perhaps the following can help you:
static void Main(string[] args)
{
string originalString= "\r\radmin#Modem -- *<456> \radmin#Modem -- *<456> ";
Regex reg = new Regex(#"-- \*<[1-9][0-9]*>");
bool isMatch = reg.IsMatch(originalString);
Console.WriteLine(isMatch);
}

You can use Regex.IsMatch using this regular expression --\\s\\*<\\d+> for matching strings like -- *<456>
bool MatchTheNumTag(string str)
{
Regex reg = new Regex("--\\s\\*<\\d+>");
return reg.IsMatch(str);
}

you can use this regex
<[1-9][0-9]*>
explanation:
[1-9]
this part is a range from 1-9 so your number is bigger than 0
[0-9]*
number range from 0-9 and the * gives you the possibility to have number as big as you want
other way:
you can also use special characters for numbers, but then it really depends on the regex syntax
\d

welcome to Regexes!
You ave to know that certain characters in regexes are special characters and they need to be escaped, you can find them here: http://www.regular-expressions.info/characters.html
Which means that a regex for your pattern would be \s\-\-\s\*<456>
\s just means whitespace.

Get specific numbers from string

In my current project I have to work alot with substring and I'm wondering if there is an easier way to get out numbers from a string.
Example:
I have a string like this:
12 text text 7 text
I want to be available to get out first number set or second number set.
So if I ask for number set 1 I will get 12 in return and if I ask for number set 2 I will get 7 in return.
Thanks!

This will create an array of integers from the string:
using System.Linq;
using System.Text.RegularExpressions;
class Program {
static void Main() {
string text = "12 text text 7 text";
int[] numbers = (from Match m in Regex.Matches(text, #"\d+") select int.Parse(m.Value)).ToArray();
}
}

Try using regular expressions, you can match [0-9]+ which will match any run of numerals within your string. The C# code to use this regex is roughly as follows:
Match match = Regex.Match(input, "[0-9]+", RegexOptions.IgnoreCase);
// Here we check the Match instance.
if (match.Success)
{
// here you get the first match
string value = match.Groups[1].Value;
}
You will of course still have to parse the returned strings.

Looks like a good match for Regex.
The basic regular expression would be \d+ to match on (one or more digits).
You would iterate through the Matches collection returned from Regex.Matches and parse each returned match in turn.
var matches = Regex.Matches(input, "\d+");
foreach(var match in matches)
{
myIntList.Add(int.Parse(match.Value));
}

You could use regex:
Regex regex = new Regex(#"^[0-9]+$");

you can split the string in parts using string.Split, and then travese the list with a foreach applying int.TryParse, something like this:
string test = "12 text text 7 text";
var numbers = new List<int>();
int i;
foreach (string s in test.Split(' '))
{
if (int.TryParse(s, out i)) numbers.Add(i);
}
Now numbers has the list of valid values

C# Why i can not split the string?

string myNumber = "3.44";
Regex regex1 = new Regex(".");
string[] substrings = regex1.Split(myNumber);
foreach (var substring in substrings)
{
Console.WriteLine("The string is : {0} and the length is {1}",substring, substring.Length);
}
Console.ReadLine();
I tried to split the string by ".", but it the splits return 4 empty string. Why?

. means "any character" in regular expressions. So don't split using a regex - split using String.Split:
string[] substrings = myNumber.Split('.');
If you really want to use a regex, you could use:
Regex regex1 = new Regex(#"\.");
The # makes it a verbatim string literal, to stop you from having to escape the backslash. The backslash within the string itself is an escape for the dot within the regex parser.

the easiest solution would be: string[] val = myNumber.Split('.');

. is a reserved character in regex. if you literally want to match a period, try:
Regex regex1 = new Regex(#"\.");
However, you're better off simply using myNumber.Split(".");

The dot matches a single character, without caring what that character
is. The only exception are newline characters.
Source: http://www.regular-expressions.info/dot.html
Therefore your implying in your code to split the string at each character.
Use this instead.
string substr = num.Split('.');

Keep it simple, use String.Split() method;
string[] substrings = myNumber.Split('.');
It has an other overload which allows specifying split options:
public string[] Split(
char[] separator,
StringSplitOptions options
)

You don't need regex you do that by using Split method of string object
string myNumber = "3.44";
String[] substrings = myNumber.Split(".");
foreach (var substring in substrings)
{
Console.WriteLine("The string is : {0} and the length is {1}",substring, substring.Length);
}
Console.ReadLine();

The period "." is being interpreted as any single character instead of a literal period.
Instead of using regular expressions you could just do:
string[] substrings = myNumber.Split(".");

In Regex patterns, the period character matches any single character. If you want the Regex to match the actual period character, you must escape it in the pattern, like so:
#"\."
Now, this case is somewhat simple for Regex matching; you could instead use String.Split() which will split based on the occurrence of one or more static strings or characters:
string[] substrings = myNumber.Split('.');

try
Regex regex1 = new Regex(#"\.");
EDIT: Er... I guess under a minute after Jon Skeet is not too bad, anyway...

You'll want to place an escape character before the "." - like this "\\."
"." in a regex matches any character, so if you pass 4 characters to a regex with only ".", it will return four empty strings. Check out this page for common operators.

Try
Regex regex1 = new Regex("[.]");

Removing numbers from text using C#

I have a text file for processing, which has some numbers. I want JUST text in it, and nothing else. I managed to remove the punctuation marks, but how do I remove the numbers? I want this using C# code.
Also, I want to remove words with length greater than 10. How do I do that using Reg Expressions?

You can do this with a regex:
string withNumbers = // string with numbers
string withoutNumbers = Regex.Replace(withNumbers, "[0-9]", "");
Use this regex to remove words with more than 10 characters:
[\w]{10, 100}
100 defines the max length to match. I don't know if there is a quantifier for min length...

Only letters and nothing else (because I see you also want to remove the punctuation marks)
Regex.IsMatch(input, #"^[a-zA-Z]+$");

You can also use string.Join:
string s = "asdasdad34534t3sdf43534";
s = string.Join(null, System.Text.RegularExpressions.Regex.Split(s, "[\\d]"));

The Regex.Replace method should do the trick.
// regex to match any digit
var regex = new Regex("\d");
// replace all matches in input with empty string
var output = regex.Replace(input, String.Empty);