Regular Expressions

PowerShell – Writing Regular Expressions

A regular expression is a string, written using a special regular expression language, that helps a computer identify strings that are of a particular format—such as an IP address, a UNC, or an e-mail address. A well-written regular expression has the ability to allow a Windows PowerShell script to accept as valid or reject as invalid data that does not conform to the format you’ve specified.

The Windows PowerShell –match operator compares a string to a regular expression and then returns either True or False depending on whether the string matches the regex.

Here is an example of a simple match

PS C:\> "WinAdmin.org" -match "WIn"
True
PS C:\> "WinAdmin.org" -match "windows"
False

By default, a regex is case-insensitive in PowerShell. But if you want to match case sensitive, you can use -cmatch

PS C:\> "WinAdmin.org" -cmatch "admin"
False
PS C:\> "WinAdmin.org" -cmatch "Admin"
True

Regular Expressions – Wildcards and Repeaters

A regex can contain a few wildcard characters.

. (period) – matches one instance of any character

? (question mark) – matches zero or one instance of any character

* matches zero or more of the specified characters

+ matches one or more of the specified characters.

PS C:\> "hello" -match "h.llo"
True
PS C:\> "hello" -match "h?ello"
True
PS C:\> "hello" -match "h?llo"
True
PS C:\> "hello" -match "h*o"
True
PS C:\> "hello" -match "hel+o"
True

Regular Expressions – Character Classes

• \w matches any word character, meaning letters and numbers.
• \s matches any white space character, such as tabs, spaces, and so forth.
• \d matches any digit character.
  

PS C:\> "Windows Server" -match "\w"
True
PS C:\> "Windows Server" -match "\w\s\w"
True
PS C:\> "Windows Server" -match "\d"
False
PS C:\> "Windows Server 2016" -match "\d"
True

Regular Expressions – Character Groups, Ranges, and Sizes

A regex can also contain groups or ranges of characters, enclosed in square brackets. For example, [aeiou] means that any one of the included characters—a, e, i, o, or u—is an acceptable match. [a-zA-Z] indicates that any letter in the range a-z or A-Z is acceptable

PS C:\> "Windows" -match "Win[def]ows"
True

Creating a regular expression to check IP.

PS C:\> "192.168.0.1" -match "\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}"
True
PS C:\> "192.168.0.A" -match "\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}"
False