How to pass values from a string array into a Regex in C#

Question

I am currently writing a program that will check a line from a file and see if a US state is contained in that line and that it is also spelled correctly. I have it currently working for a single state. I would like to be able to see if any of all the US states are in the line.

This is my code so far below

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using System.IO;

namespace ConsoleApplication3
{
class Program
{
    static void Main(string[] args)
    {

        var r = new Regex(@"\bArizona\b", RegexOptions.IgnoreCase);
        string[] lines = File.ReadAllLines(@"C:\sampledata.dat");
        foreach (string s in lines)
        {
            var m = r.Match(s);
            Console.WriteLine(m.Success); // false
            Console.WriteLine(s);
            Console.ReadLine();
        }
    }
}
}

Essentially I would like to do something like var r = new Regex(@"\bAll US States.txt\b", RegexOptions.IgnoreCase);

You can try loading all of the states from your file into an array or list and loop through each one as a new regex and do your check. — tastydew
– tastydew, Commented May 12, 2017 at 15:03
Answered at stackoverflow.com/questions/1209049/c-regex-match-whole-words — Andrew Paes
– Andrew Paes, Commented May 12, 2017 at 15:04
@ephtee Running 50 regular expressions instead of one seems like a bad idea. — juharr
– juharr, Commented May 12, 2017 at 15:05
@juharr No doubt. But given the nature of the question this seems like it's probably a homework assignment and therefore an acceptable approach which will teach him/her to consider better ways to do it. — sab669
– sab669, Commented May 12, 2017 at 15:07
Hi guys, trying to write this program to automate a process in work. At the moment someone has to manually check the file to make sure all states are spelled correctly. There can be up to 1000 lines in each file. — user3394486
– user3394486, Commented May 12, 2017 at 15:11

Graham · Accepted Answer · 2017-09-29 14:39:02Z

3

If you wanted to see if any of the states were contained, you could essentially use the string.Join() method to generate an expression that would match any of them:

// Read your lines
var states = File.ReadAllLines(@"C:\states.txt");

// Build a pipe delimited string (e.g. State1|State2|State3 ...) to use an a Regex with necessary boundaries
// leading and trailing boundaries
var pattern = $@"\b{(string.Join("\b|\b", states))}\b";

// Now use that pattern to build a Regex to check against (using C# string interpolation)
var regex = new Regex(pattern, RegexOptions.IgnoreCase);

// Now loop through your data here and check each line
string[] lines = File.ReadAllLines(@"C:\sampledata.dat");
foreach (string s in lines)
{
        var m = regex.Match(s);
        Console.WriteLine(m.Success); // false
        Console.WriteLine(s);
        Console.ReadLine();
}

Additionally, if you aren't able to use string interpolation to build your pattern, simply use the older string.Format() approach:

var pattern = string.Format("\b{0}\b", string.Join("\b|\b", states));

edited Sep 29, 2017 at 14:39

Graham

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answered May 12, 2017 at 15:03

Rion Williams

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2 Comments

juharr Over a year ago

Don't forget to add the \b on each end.

Dmitry Egorov Over a year ago

This pattern would match New Yorker, which is not what OP would expect, I believe.

Dmitry Egorov · Accepted Answer · 2017-05-12 15:14:55Z

2

The regex you're looking for is

\b(?:Alabama|Alaska|Arizona|...)\b

To compose it from a list stored in an All US States.txt file use File.ReadAllLines:

var states = File.ReadAllLines(@"All US States.txt");
var r = new Regex(string.Format(@"\b(?:{0})\b", string.Join("|", states)), RegexOptions.IgnoreCase);

edited May 12, 2017 at 15:14

answered May 12, 2017 at 15:05

Dmitry Egorov

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3 Comments

user3394486 Over a year ago

This worked for me, however what does the ? in (@"\b(?:{0})\b" do?

Dmitry Egorov Over a year ago

(?:...) is a non-capturing group. This allows to ensure that word boundaries (\b) are applied to any state from the alternation list.

C Perkins Over a year ago

@DmitryEgorov I like the solution, but (?:...) is equivalent to plain parenthesis ( ), the only difference that it does not store the matched value for later reference. It does not change the matching behavior of the parenthesis, so ?: is not required to have \b applied correctly.

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