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How to structure REGEX in C#

I currently have a regex that checks if a US State is spelled correctly
var r = new Regex(string.Format(#"\b(?:{0})\b", pattern), RegexOptions.IgnoreCase)
pattern is a pipe delimited string containing all US states.
It was working as intended today until one of the states was spelled like "Florida.." I would have liked it picked up the fact there was a fullstop character.
I found this regex that will only match letters.
^[a-zA-Z]+
How do I combine this with my current Regex or is it not possible?
I tried some variations of this but it didn't work
var r = new Regex(string.Format(#"\b^[a-zA-Z]+(?:{0})\b", pattern), RegexOptions.IgnoreCase);
EDIT: Florida.. was in my input string. My pattern string hasn't changed at all. Apologies for not being clearer.

It seems you need start of string (^) and end of string ($) anchors:
var r = new Regex(string.Format(#"^(?:{0})$", pattern), RegexOptions.IgnoreCase);
The regex above would match any string comprising a name of a state only.

You should make a replacement of the pattern variable to escape the regex special characters. One of them is the . character. Something similar to pattern.Replace(".", #"\.") but doing all the especial characters.

I believe you can't merge both patterns into one, so you would have to perform two diferent regex operations, one to split the states into a list, and a subsequent one for the validation of each item within it.
I'd rather go for something "simpler" such as
var states = input.Split('|').Select(s => new string(s.Where(char.IsLetter).ToArray()))
.Where(s => !string.IsNullOrWhiteSpace(s));

Basically don't use a regex here.
List<string> values = new List<string>() {"florida", etc.};
string input;
//is input in values, ignore case and look for any value that includes the input value
bool correct = values.Any(a =>
input.IndexOf(a, StringComparison.CurrentCultureIgnoreCase) >= 0);
This will be considerably more efficient than a regex based option. This should match florida, Florida and Florida..., etc.

Don't search for characters directly, tell regex to consume all which are not targeted specific characters such as [^\|.]+. It uses the set [ ] with the not ^ indicator says consume anything which is not a literal | or .. Hence it consumes just the text needed. Such as on
Colorado|Florida..|New Mexico
returns 3 matches of Colorado Florida and New Mexico

c# Regex: find placeholders as substring

i have following string.
"hello [#NAME#]. nice to meet you. I heard about you via [#SOURCE#]."
in above text i have two place holders. NAME and SOURCE
i want to extract these sub string using Reg Ex.
what would be the reg ex pattern to find list of these place holders.
i tried
string pattern = #"\[#(\w+)#\]";
result
hello
NAME
. nice to meet you. I heard about you via
SOURCE
.
what i want is only
NAME
SOURCE
Sample code
string tex = "hello [#NAME#]. nice to meet you. I heard about you via [#SOURCE#].";
string pattern = #"\[#(\w+)#\]";
var sp = Regex.Split(tex, pattern);
sp.Dump();

Your regex is working correctly. That's, how Regex.Split() should behave (see the doc). If what you said is really what you want, you can use something like:
var matches = from Match match in Regex.Matches(text, pattern)
select match.Groups[1].Value;
If, on the other hand, you wanted to replace the placeholders using some rules (e.g. using a Dictionary<string, string>), then you could do:
Regex.Replace(text, pattern, m => substitutions[m.Groups[1].Value]);

Try this regex:
\[#([A-Z]+)#\]

^hello (.*?). nice to meet you. I heard about you via (.*?).$
Very simply, the () means you want to capture what's inside, the .*? is (what's known as) an "ungreedy" capture (capture as few characters as possible). and . means any character.
demo of above
Unless you're placeholds are always going to use [# prefix, and #] postfix, then see the other users' posts.

How to get numbers from http://www.example.com/images/business/113.jpg

Using regex I need to get the numbers between the last "/" and ".jpg" (this actually might be .png, .gif, etc) in this:
http://www.example.com/images/business/113.jpg
Any ideas?
Thank you

Easy enough using split:
var fileName = myUrl.Split('/')[myUrl.Split('/').Length - 1];
var justTheFileName = fileName.Split('.')[0];

Regular expression are absolute unnecessary here.
Just do:
using System.IO;
var fileName = Path.GetFileNameWithoutExtension("http://www.example.com/images/business/113.jpg");
Take a look at the documentation of the method GetFileNameWithoutExtension:
Returns the file name of the specified path string without the extension.
Edit:
If you still want to use regex for this purpose, the following one will work:
//Both regexes will work here
var pattern = #"/([^/]*)\.jpg"
var pattern2 = #".*/(.*)\.jpg"
var matches = Regex.Matches(pattern, "http://www.example.com/images/business/113.jpg");
if (matches.Count > 0)
Console.WriteLine(matches[0].Groups[1].Count);
Note:
I didn't compile the regex. This was a small & fast example.

I see that you found a solution matches a single digit in your URL 3 times, but not the entire number. You may want to go with something more "readable" (heh) like this:
(?<=\/)\d+(?=\.\w+$)
If you're trying to capture the number and use it, throw it into a group:
(?<=\/)(\d+)(?=\.\w+$)

Got it!! (?=[\s\S]*?\\.)(?![\s\S]+?/)[0-9]
PS: The regular expression workbench by microsoft KICKS ASS

You could use the following regular expression:
/(?<number>\d+)\.jpg$
It will capture the number into the named group 'number'. The regular expression works as follows:
Search for /
Capture 1 or more times a digit (0-9) to the named group 'number'
Check for .jpg
$ matches the end of the string.
Matching the end makes stuff a lot easier. I don't believe look-ahead or look-behind is necessary.

Convert C# regex Code to Java

I have found this Regex extractor code in C#.
Can someone tell me how this works, and how do I write the equivalent in Java?
// extract songtitle from metadata header.
// Trim was needed, because some stations don't trim the songtitle
fileName =
Regex.Match(metadataHeader,
"(StreamTitle=')(.*)(';StreamUrl)").Groups[2].Value.Trim();

This should be what you want.
// Create the Regex pattern
Pattern p = Pattern.compile("(StreamTitle=')(.*)(';StreamUrl)");
// Create a matcher that matches the pattern against your input
Matcher m = p.matcher(metadataHeader);
// if we found a match
if (m.find()) {
// the filename is the second group. (The `(.*)` part)
filename = m.group(2);
}

It pulls "MyTitle" from a string such as "StreamTitle='MyTitle';StreamUrl".
The () operators define match groups, there are 3 in your regex. The second one contains the string of interest, and is gotten in the Groups[2].Value.
There's a few very good regex designers out there. The one I use is Rad Software's Regular Expression Designer (www.radsoftware.com.au). It is very useful for figuring out stuff like this (and it uses C# RegEx's).

C# reliable way to pattern match?

At the moment I am trying to match patterns such as
text text date1 date2
So I have regular expressions that do just that. However, the issue is for example if users input data with say more than 1 whitespace or if they put some of the text in a new line etc the pattern does not get picked up because it doesn't exactly match the pattern set.
Is there a more reliable way for pattern matching? The goal is to make it very simple for the user to write but make it easily matchable on my end. I was considering stripping out all the whitespace/newlines etc and then trying to match the pattern with no spaces i.e. texttextdate1date2.
Anyone got any better solutions?
Update
Here is a small example of the pattern I would need to match:
FIND me#test.com 01/01/2010 to 10/01/2010
Here is my current regex:
FIND [A-Z0-9._%+-]+#[A-Z0-9.-]+\.[A-Z]{2,4} [0-9]{1,2}/[0-9]{1,2}/[0-9]{2,4} to [0-9]{1,2}/[0-9]{1,2}/[0-9]{2,4}
This works fine 90% of the time, however, if users submit this information via email it can have all different kinds of formatting and HTML I am not interested in. I am using a combination of the HtmlAgilityPack and a HTML tag removing regex to strip all the HTML from the email, but even at that I can't seem to get a match on some occassions.
I believe this could be a more parsing related question than pattern matching, but I think maybe there is a better way of doing this...

To match at least one or more whitespace characters (space, tab, newline), use:
\s+
Substitute the above wherever you have the physical space in your pattern and you should be fine.

Example of matching multiple groups in a text with multiple whitespaces and/or newlines.
var txt = "text text date1\ndate2";
var matches = Regex.Match(txt, #"([a-z]+)\s+([a-z]+)\s+([a-z0-9]+)\s+([a-z0-9]+)", RegexOptions.Singleline);
matches.Groups[n].Value with n from 1 to 4 will contain your matches.

I would split the string into a string array and match each resulting string to the necessary Regular Expression.

\b(text)[\s]+(text)[\s]+(date1)[\s]+(date2)\b

Its a nasty expression but here is something that will work for the input you provided:
^(\w+)\s+([\w#.]+)\s+(\d{2}\/\d{2}\/\d{4})[^\d]+(\d{2}\/\d{2}\/\d{4})$
This will work with variable amounts of whitespace between the capture groups as well.

Through ORegex you can tokenize your string and just pattern match on token sequences:
var tokens = input.Split(new[]{' ','\t','\n','\r'}, StringSplitOptions.RemoveEmptyEntries);
var oregex = new ORegex<string>("{0}{0}{1}{1}", IsText, IsDate);
var matches = oregex.Matches(tokens); //here is your subsequence tokens.
...
public bool IsText(string str)
{
...
}
public bool IsDate(string str)
{
...
}