About Regular Expressions
Short description
Describes regular expressions in PowerShell.
Long description
Note
This article will show you the syntax and methods for using regular expressions in PowerShell, not all syntax is discussed. For a more complete reference, see the Regular Expression Language - Quick Reference.
A regular expression is a pattern used to match text. It can be made up of literal characters, operators, and other constructs.
This article demonstrates regular expression syntax in PowerShell. PowerShell has several operators and cmdlets that use regular expressions. You can read more about their syntax and usage at the links below.
PowerShell regular expressions are case-insensitive by default. Each method shown above has a different way to force case sensitivity.
| Method | Case Sensitivity |
|---|---|
Select-String |
use -CaseSensitive switch |
switch statement |
use the -casesensitive option |
| operators | prefix with 'c' (-cmatch, -csplit, or -creplace) |
Character literals
A regular expression can be a literal character or a string. The expression causes the engine to match the text specified exactly.
# This statement returns true because book contains the string "oo"
"book" -match "oo"
Character classes
While character literals work if you know the exact pattern, character classes allow you to be less specific.
Character groups
[character group] allows you to match any number of characters one time,
while [^character group] only matches characters NOT in the group.
# This expression returns true if the pattern matches big, bog, or bug.
"big" -match "b[iou]g"
If your list of characters to match includes the hyphen character (-), it
must be at the beginning or end of the list to distinguish it from a character
range expression.
Character ranges
A pattern can also be a range of characters. The characters can be alphabetic [A-Z],
numeric [0-9], or even ASCII-based [ -~] (all printable characters).
# This expression returns true if the pattern matches any 2 digit number.
42 -match "[0-9][0-9]"
Numbers
The \d character class will match any decimal digit. Conversely, \D will
match any non-decimal digit.
# This expression returns true if it matches a server name.
# (Server-01 - Server-99).
"Server-01" -match "Server-\d\d"
Word characters
The \w character class will match any word character [a-zA-Z_0-9]. To match
any non-word character, use \W.
# This expression returns true.
# The pattern matches the first word character 'B'.
"Book" -match "\w"
Wildcards
The period (.) is a wildcard character in regular expressions. It will match
any character except a newline (\n).
# This expression returns true.
# The pattern matches any 4 characters except the newline.
"a1\ " -match "...."
Whitespace
Whitespace is matched using the \s character class. Any non-whitespace
character is matched using \S. Literal space characters ' ' can also be
used.
# This expression returns true.
# The pattern uses both methods to match the space.
" - " -match "\s- "
Quantifiers
Quantifiers control how many instances of each element should be present in the input string.
The following are a few of the quantifiers available in PowerShell:
| Quantifier | Description |
|---|---|
* |
Zero or more times. |
+ |
One or more times. |
? |
Zero or one time. |
{n,m} |
At least n, but no more than m times. |
The asterisk (*) matches the previous element zero or more times. The result
is that even an input string without the element would be a match.
# This returns true for all account name strings even if the name is absent.
"ACCOUNT NAME: Administrator" -match "ACCOUNT NAME:\s*\w*"
The plus sign (+) matches the previous element one or more times.
# This returns true if it matches any server name.
"DC-01" -match "[A-Z]+-\d\d"
The question mark ? matches the previous element zero or one time. Like
asterisk *, it will even match strings where the element is absent.
# This returns true for any server name, even server names without dashes.
"SERVER01" -match "[A-Z]+-?\d\d"
The {n, m} quantifier can be used several different ways to allow granular
control over the quantifier. The second element m and the comma , are
optional.
| Quantifier | Description |
|---|---|
{n} |
Match EXACTLY n number of times. |
{n,} |
Match at LEAST n number of times. |
{n,m} |
Match between n and m number of times. |
# This returns true if it matches any phone number.
"111-222-3333" -match "\d{3}-\d{3}-\d{4}"
Anchors
Anchors allow you to cause a match to succeed or fail based on the matches position within the input string.
The two commonly used anchors are ^ and $. The caret ^ matches the start
of a string, and $, which matches the end of a string. The anchors allow you
to match your text at a specific position while also discarding unwanted
characters.
# The pattern expects the 'h' to be followed by the end of the word.
# This will return FALSE.
"fishing" -match "^fish$"
When using anchors in powershell, you should understand the difference between Singleline and Multiline regular expression options.
- Multiline: Multiline mode forces
^and$to match the beginning end of every LINE instead of the beginning and end of the input string. - Singleline: Singleline mode treats the input string as a SingleLine.
It forces the
.character to match every character (including newlines), instead of matching every character EXCEPT the newline\n.
To read more about these options and how to use them, visit the Regular Expression Language - Quick Reference.
Escaping characters
The backslash (\) is used to escape characters so they aren't parsed by the
regular expression engine.
The following characters are reserved: []().\^$|?*+{}.
You'll need to escape these characters in your patterns to match them in your input strings.
# This returns true and matches numbers with at least 2 digits of precision.
# The decimal point is escaped using the backslash.
"3.141" -match "3\.\d{2,}"
There`s a static method of the regex class that can escape text for you.
[regex]::escape("3.\d{2,}")
3\.\\d\{2,}
Note
This escapes all reserved regular expression characters, including existing backslashes used in character classes. Be sure to only use it on the portion of your pattern that you need to escape.
Other character escapes
There are also reserved character escapes that you can use to match special character types.
The following are a few commonly used character escapes:
| Character Escape | Description |
|---|---|
\t |
Matches a tab |
\n |
Matches a newline |
\r |
Matches a carriage return |
Groups, Captures, and Substitutions
Grouping constructs separate an input string into substrings that can be captured or ignored. Grouped substrings are called subexpressions. By default subexpressions are captured in numbered groups, though you can assign names to them as well.
A grouping construct is a regular expression surrounded by parentheses. Any text matched by the enclosed regular expression is captured. The following example will break the input text into two capturing groups.
"The last logged on user was CONTOSO\jsmith" -match "(.+was )(.+)"
True
Use the $Matches Hashtable automatic variable to retrieve captured text.
The text representing the entire match is stored at key 0.
$Matches.0
The last logged on user was CONTOSO\jsmith
Captures are stored in numeric Integer keys that increase from left to
right. Capture 1 contains all the text until the username, capture 2
contains just the username.
$Matches
Name Value
---- -----
2 CONTOSO\jsmith
1 The last logged on user was
0 The last logged on user was CONTOSO\jsmith
Important
The 0 key is an Integer. You can use any Hashtable method to access
the value stored.
PS> "Good Dog" -match "Dog"
True
PS> $Matches[0]
Dog
PS> $Matches.Item(0)
Dog
PS> $Matches.0
Dog
Named Captures
By default, captures are stored in ascending numeric order, from left to right.
You can also assign a name to a capturing group. This name becomes a
key on the $Matches Hashtable automatic variable.
Inside a capturing group, use ?<keyname> to store captured data under a named
key.
PS> $string = "The last logged on user was CONTOSO\jsmith"
PS> $string -match "was (?<domain>.+)\\(?<user>.+)"
True
PS> $Matches
Name Value
---- -----
domain CONTOSO
user jsmith
0 was CONTOSO\jsmith
PS> $Matches.domain
CONTOSO
PS> $Matches.user
jsmith
The following example stores the latest SuccessAudit from the Windows Security Log. The provided regular expression extracts the username and domain from the message and stores them under the keys: N for name and D for domain.
$log = (Get-EventLog -LogName Security -Newest 1 -InstanceId 4689).message
$r = '(?s).*Account Name:\s*(?<N>.*).*Account Domain:\s*(?<D>[A-Z,0-9]*)'
$log -match $r
True
$Matches
Name Value
---- -----
D CONTOSO
N jsmith
0 A process has exited....
For more information, see Grouping Constructs in Regular Expressions.
Substitutions in Regular Expressions
Using the regular expressions with the -replace operator allows you to
dynamically replace text using captured text. Capturing groups can be
referenced in the <substitute> string. The substitution is done by using the
$ character before the group identifier.
<input> -replace <original>, <substitute>
Two ways to reference capturing groups are by Number and by Name.
By Number - Capturing Groups are numbered from left to right.
"John D. Smith" -replace "(\w+) (\w+)\. (\w+)", '$1.$2.$3@contoso.com'John.D.Smith@contoso.comBy Name - Capturing Groups can also be referenced by name.
"CONTOSO\Administrator" -replace '\w+\\(?<user>\w+)', 'FABRIKAM\${user}'FABRIKAM\Administrator
The $& expression represents all the text matched.
"Gobble" -replace "Gobble", "$& $&"
Gobble Gobble
Warning
Since the $ character is used in string expansion, you'll need to use
literal strings with substitution, or escape the $ character.
'Hello World' -replace '(\w+) \w+', "`$1 Universe"
Hello Universe
Additionally, since the $ character is used in substitution, you will need
to escape any instances in your string.
'5.72' -replace '(.+)', '$$$1'
$5.72
For more information, see Substitutions in Regular Expressions.