Regular expressions are incredibly powerful, but they're sometimes described as "looking like cartoon characters swearing", and the syntax can be difficult to remember.

I only find myself needing to write code to "capture" and replace values from a string a couple of times a year and always have to re-learn the syntax, so I'm blogging this for my reference. I hope it will be helpful to you also.

I'm including examples in JavaScript and C#...

Here's the scenario for the following examples: We want to log XML payment account setup requests/responses, but the request contains a bank account number:

<Name>Sparky's Bank Account</Name>
<BankRoutingNumber>123456789</BankRoutingNumber>
<BankAccountNumber>987654321</BankAccountNumber>

Bank account numbers are sensitive information, so we don't want to log them as-is.

Here's the basic Regex to find BankAccountNumber XML elements. The "\d+" (one or more digits) pattern defines the account number:

JavaScript

const regex =
    new RegExp('<BankAccountNumber>\\d+<\/BankAccountNumber>', 'g');

var regex = new Regex(
    @"<BankAccountNumber>\d+<\/BankAccountNumber>", 
    RegexOptions.Compiled);

...and code using the Regex to find the bank account XML elements:

JavaScript

const matches = requestXml.matchAll(regex);
console.log([...matches]);

foreach (Match match in regex.Matches(requestXml))
{
    Console.WriteLine(match.Value);
}

Results:

<BankAccountNumber>987654321</BankAccountNumber>

Replacing

Let's replace the bank account numbers with asterisks:

JavaScript

const censored = requestXml.replace(
    regex,
    '<BankAccountNumber>*********</BankAccountNumber>');

console.log(censored);

string censored = regex.Replace(
    requestXml,
    "<BankAccountNumber>*********</BankAccountNumber>");

Console.WriteLine(censored);

Results:

<Name>Sparky's Bank Account</Name>
<BankRoutingNumber>123456789</BankRoutingNumber>
<BankAccountNumber>*********</BankAccountNumber>

Capturing

A capture group is defined by enclosing part of the regex ("\d+" in this example) in parentheses:

<BankAccountNumber>(\\d+)<\/BankAccountNumber>

The entire match (the XML element in this example) is an automatic capture group, so this will result in two capture groups:

JavaScript

const matches = requestXml.matchAll(regex);
for (const match of matches) {
    const captures = [...match];
    for (var i = 0; i < captures.length; i++) {
        console.log(`[${i}] ${captures[i]}`)
    }
}

foreach (Match match in regex.Matches(requestXml))
{
    for (int i = 0; i < match.Groups.Count; i++)
    {
        Console.WriteLine($"[{i}] {match.Groups[i]}");
    }
}

Results:

[0] <BankAccountNumber>987654321</BankAccountNumber>
[1] 987654321

Named captures

You can name a capture group with the syntax "(?<name>pattern)". In this example, I'm using the name "acctNum":

<BankAccountNumber>(?<acctNum>\\d+)<\/BankAccountNumber>

(The angle bracket syntax for group naming is a bit confusing for this example because it looks like XML.)

Using a named capture:

JavaScript

const matches = requestXml.matchAll(regex);
for (const match of matches) {
    console.log(match.groups.acctNum);
}

foreach (Match match in regex.Matches(requestXml))
{
    Console.WriteLine(match.Groups["acctNum"].Value);
}

Results:

987654321

Replacing with a function

We shouldn't log account numbers, but let's say our security policy allows logging them "masked". Let's try replacing with a "callback" function that replaces all but the last four digits with asterisks:

JavaScript

const censored = requestXml.replaceAll(regex, match => {
    const innerMatches = [...match.matchAll(regex)];
    const acctNum = innerMatches[0].groups.acctNum;
    const len = acctNum.length;
    const masked = (len > 4)
        ? '*'.repeat(len - 4) + acctNum.substr(len - 4)
    : '*'.repeat(len);
    return `<BankAccountNumber>${masked}</BankAccountNumber>`;
});

console.log(censored);

string censored = regex.Replace(requestXml, match =>
{
    string accountNumber = match.Groups["acctNum"].Value;
    int len = accountNumber.Length;
    string masked = (len > 4) 
        ? new string('*', len - 4) + accountNumber.Substring(len - 4) 
        : new string('*', len);
    return $"<BankAccountNumber>{masked}</BankAccountNumber>";
});

Results:

<Amount>123.45</Amount>
<BankRoutingNumber>123456789</BankRoutingNumber>
<BankAccountNumber>*****4321</BankAccountNumber>

.Net "Fiddle" containing the C# code excerpted above

JS "Fiddle" containing the JavaScript code excerpted above

Blog

Regular expression capture and replace examples

Brian Schroer

Replacing

Capturing

Named captures

Replacing with a function

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