Regular Expressions for Beginners: How to Get Started Discovering Sensitive Data

Any data discovery and classification solution heavily relies on regular expressions (sometimes called RegExes, REs or RegEx patterns) to identify sensitive data. But what are RegExes and how can they be used to discover sensitive data? Let’s find out.

Regular expressions are a small but highly specialized programming language; they are basically wildcards on steroids. Using this little language, you specify rules that define the strings you want to match. For example, you can define a RegEx that will match email addresses, PII, PHI or credit card numbers.

Regex Components

A RegEx can include literals and metacharacters.


Any single character, except for those reserved as metacharacters, is already a regular expression itself. For example, www is a match for www.netwrix.com but wwz is not. Note that regular expressions are case sensitive, so www will not match WWW or wWw.


The following single characters are not interpreted as literals but instead have special meanings:

  • . ^ $ * + ? { } [ ] \ | ( )

The following table describes how each of these metacharacters functions.

Type Meta-characters Description Examples  
The dot . The period means any character. net.rix will match both www.netwrix.com and www.netfrix.com.
Character class [] Matches for anything inside the square brackets.
The one exception is the ^ character. Inside a class, at the beginning, the ^ means exception from the search. For example [^n] will match any character except n; this is called a negated character class.
Note that metacharacters (with one exception) are not active inside classes. For example, [net$] will match any of the characters n, e, t or $ ($ is a metacharacter, but inside a character class it matches only $).
The one exception is the ^ character. Inside a class, at the beginning, the ^ means exception from the search. For example [^n] will match any character except n; this is called a negated character class.
You can list characters individually; for instance, net[wrx] will match netw , netr and netx but not netz.
Or you can look for a range of characters by giving two characters and separating them by a hyphen; for example, net[a-z] will match neta, netw and netf but not net1.
Anchors ^ Used to match characters at the beginning of a string ^https will match https://netwrix.com but not www.netwrix.com or http://netwrix.com
$ Used to match characters at the end of a string com$ will match www.netwrix.com or telecom but not computer.
Iteration / quantifiers ? Matches the preceding element zero or one time (it will always match if the character was not found). It is great for finding optional characters. colou?r will match both color and colour.
* Matches the preceding element zero or more times instead of zero or once. It is great for finding optional series of characters. ne*t will match nt (zero e characters), net (one e ), neeet (three e characters), and so forth.
+ Matches the preceding element one or more times.
Pay careful attention to the difference between * and +. * matches zero or more times, so whatever’s being repeated may not be present at all; + requires at least one occurrence.
ne+t will match net and neeet but not nt.
| The choice operator matches either the expression before or the expression after the operator. net|wrix will match net and wrix.
{} {x} matches if the element that precedes it is found exactly x times.
{x,y} matches if the preceding element is found at least x times but not more than y times.
n{3} will match nnn , nnnn and nnnd (because they all include n three times in a row), but it will not match nnw.
9{3} will match 999, 1234999124 and text999text, but not 84299238, 9909, or page992.
n{3,5} will match nnn, nnnn and nnnnn.
Blocking and capturing () Defines a subexpression that can be recalled later using shorthand: The first subexpression in parentheses can be recalled by \1, the second can be recalled by \2 and so on.
Parentheses are normally used either with | (the choice operator) inside or with quantifiers on the outside.
Gr(a|e)y will match Gray or Grey.
[0-9]([-])[0-9]\1[0-9] will match 3-4-2 and 4-6-1, but not 1-23, 42-1 or 234.
Escape sequence \ The metacharacter that follows the slash will be used as a literal.
Note that some sequences beginning with \ are not escape sequences. Instead, they represent predefined sets of characters that are often useful, such as the set of digits, the set of letters, or the set of anything that isn’t whitespace. The most popular ones are listed below as “special metacharacters.”
www\.netwrix\.com will match www.netwrix.com but not www,netwrix,com.
Special metacharacters \s Matches any whitespace character (a space, a tab, a line break or a form feed). Netwrix\sAuditor will match Netwrix Auditor, and Netwrix(tab)Auditor, but not Netwrix Auditor or NetwrixAuditor.
\S Matches any non-whitespace character. \Snetwrix will match Xnetwrix and 1netwrix.
\w Matches any alphanumeric character. \w\w\w will match net, dfw and Netwrix.
\W Matches any non-alphanumeric character. netwrix\W will match netwrix! and netwrix?.
\d Matches any decimal digit. Netwrix\d\d will match Netwrix80 and Netwrix90.
\D Matches any non-digit character. Netwrix\D will match Netwrix) and Netwrix-.
\a Matches any single alphabetic character, either capital or lowercase. net\arix will match netWrix, netfrix and netarix.
\b Defines a word boundary. \brix will match rix and rixon but not netwrix.
\B Defines a non-word boundary. \Brix will match Netwrix and trix but not rixon.

Metacharacter combinations

Now we know almost all the metacharacters and are ready to combine them.

Example: Looking for license plate numbers

Suppose we need to find a license number in the format aaa-nnnn — the first three digits must be alphanumeric and the last four must be numeric. The hyphen can be replaced with any character or missing altogether.

The RegEx for this will be:

  • \b[0-9A-Z]{3}([^ 0-9A-Z]|\s)?[0-9]{4}\b

Let’s dissect this RegEx:

  • \b requires a word boundary, so matching strings cannot be part of a larger string.
  • [0-9A-Z]{3} means that the first three characters must be alphanumeric.
  • ([^ 0-9A-Z]|\s)? means the next part of the string must be either a delimiter — a non-alphanumeric character or a whitespace character — or nothing at all.
  • [0-9]{4} means the next part of the string must be 4 digits.
  • \b specifies another word boundary.

This RegEx will match the following license numbers: NT5-6345, GH3 9452, XS83289

However, it will not match these license numbers: ZNT49371, HG3-29347, nt4-9371

Example: Looking for Social Security numbers

Another good example is U.S. Social Security number (SSN), which always takes the form nnn-nn-nnnn.

The easiest RegEx is the following:

  • [0-9]{3}-[0-9]{2}-[0-9]{4}

However, this will generate false positives, since not all numbers that have this form are legitimate SSNs. Moreover, it will miss some actual SSNs, including any that are written without the hyphens. To get more accurate results, we should build more complex one. We know that:

  • No digit group can be all zeroes.
  • The first block cannot be 666 or 900-999.
  • SSNs can be written with whitespace characters instead of hyphens, or without any delimiters at all.
  • If the first block starts with a 7, it must be followed by a number between 0 and 6 and then any third digit.

Therefore, the advanced RegEx will look like this:

  • \b(?!000|666|9\d{2})([0-8]\d{2}|7([0-6]\d))([-]?|\s{1})(?!00)\d\d\2(?!0000)\d{4}\b

As before, \b at the beginning and end specify a word boundary. Let’s look more deeply at each number block in between.

The first block

  • (?!000|666|9\d{2}) is a negative look-ahead that specifies the number must not begin with 000, 666, or 9 followed by any two digits.
  • ([0-8]\d{2} specifies that the string has to start with a digit between 0 and 8 and have two more digits (0-9) after it.
  • |7[0-6]\d)) says that it happens to begin with 7, the next digit must be between 0 and 6, followed by any digit.
  • ([-]?|\s{1}) specifies that after the three digits, there should be either a hyphen, a whitespace character or nothing at all to mark the end of the first block.

The second block

  •  (?!00) is another negative look-ahead that specifies there must not be 00 in the second block.
  • \d\d specifies that there must be any two digits in the second block.
  • \2 matches the same text as the second capturing group, which is ([-]?|\s{1}), so it specifies that the second block can end with a hyphen, a whitespace character or no additional character at all.

The third block

  • (?!0000) is another negative look-ahead that specifies there cannot be four zeroes in the third block.
  • \d{4} requires any four digits in the third SSN block.

Examples of popular RegExes

To find Use this RegEx Example of match
Email addresses ^[\w\.=-]+@[\w\.-]+\.[\w]{2,3}$ T.Simpson@netwrix.com
U.S. Social Security numbers \b(?!000|666|9\d{2})([0-8]\d{2}|7([0-6]\d))([-]?|\s{1})(?!00)\d\d\2(?!0000)\d{4}\b 513-84-7329
IPV4 addresses ^\d{1,3}[.]\d{1,3}[.]\d{1,3}[.]\d{1,3}$
Dates in MM/DD/YYYY format ^([1][12]|[0]?[1-9])[\/-]([3][01]|[12]\d|[0]?[1-9])[\/-](\d{4}|\d{2})$ 05/05/2018
MasterCard numbers ^(?:5[1-5][0-9]{2}|222[1-9]|22[3-9][0-9]|2[3-6][0-9]{2}|27[01][0-9]|2720)[0-9]{12}$ 5258704108753590
Visa card numbers \b([4]\d{3}[\s]\d{4}[\s]\d{4}[\s]\d{4}|[4]\d{3}[-]\d{4}[-]\d{4}[-
American Express card numbers ^3[47][0-9]{13}$ 34583547858682157
U.S. ZIP codes ^((\d{5}-\d{4})|(\d{5})|([A-Z]\d[A-Z]\s\d[A-Z]\d))$ 97589
File paths \\[^\\]+$ \\fs1\shared
URLs (?i)\b((?:[a-z][\w-]+:(?:\/{1,3}|[a-z0-9%])|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}\/)(?:[^\s()]+|\(([^\s()]+|(\([^\s()]+\)))*\))+(?:\(([^\s()]+

Helpful Regex web resources

Jeff is a Director of Global Solutions Engineering at Netwrix. He is a long-time Netwrix blogger, speaker, and presenter. In the Netwrix blog, Jeff shares lifehacks, tips and tricks that can dramatically improve your system administration experience.
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