I want to Select-String parts of a file path starting at a string value that is contained in a variable. Let me explain this in an abstracted example.
Let's assume this path: /docs/reports/test reports/document1.docx
Using a regular expression I can get the required string like so: '^.*(?=\\/test\\s)'
https://regex101.com/r/6mBhLX/5
The resulting string is '/test reports/document1.docx'.
Now, for this to work I have to use the literal string 'test'. However, I would like to know how to use a variable that contains 'test', eg $myString.
I already looked at How do you use a variable in a regular expression? , but I couldn't figure out how to adapt this for PowerShell.
I suggest using $([regex]::escape($myString))
inside a double quoted string literal:
$myString="[test]"
$pattern = "^.*(?=/$([regex]::escape($myString))\s)"
Or, in case you do not want to worry with additional escaping, use a regular concatenation using +
operator:
$pattern = '^.*(?=/' + [regex]::escape($myString) +'\s)'
The resulting $pattern
will look like ^.*(?=/\\[test]\\s)
. Since the $myString
variable is a literal string, you need to escape all special regex metacharacters (with [regex]::escape()
) that may be inside it for the regex engine to interpret it as literal chars.
In your case, you may use
$s = '/docs/reports/test reports/document1.docx'
$myString="test"
$pattern = "^.*(?=/$([regex]::escape($myString))\s)"
$s -replace $pattern
Result: /test reports/document1.docx
Wiktor Stribiżew's helpful answer provides the crucial pointer:
Use [regex]::Escape()
in order to escape a string for safe inclusion in a regex (regular expression) so that it is treated as a literal ;
eg, [regex]::Escape('$10?')
yields \\$10\\?
- the characters with special meaning to a regex were \\
-escaped.
However, I suggest using '...'
, ie, building the regex from single -quoted strings:
$myString='test'
$regex = '^.*(?=/' + [regex]::escape($myString) + '\s)'
Using the -f
operator - $regex = '^.*(?=/{0}'\\s)' -f [regex]::Escape($myString)
works too and is perhaps visually cleaner, but note that -f
- unlike string concatenation with +
- is culture- sensitive , which can lead to different results.
Using '...'
strings in regex contexts in PowerShell is a good habit to form :
By avoiding "..."
, you avoid additional up-front interpretation (interpolation aka expansion) of the string, which can have unexpected effects, given that $
has special meaning in both contexts: the start of a variable reference or subexpression when string-expanding, and the end-of-input marker in regexes.
Using "..."
can be especially tricky in the replacement string of the regex-based -replace
operator, where tokens such as $1
refer to capture-group results, and if you used "$1"
, PowerShell would try to expand a $1
variable , which presumably doesn't exist, resulting in the empty string .
Just write the variable within double quotes ("pattern"), like this:
PS > $pattern = "^\d+\w+"
PS > "357test*&(fdnsajkfj" -match $pattern # return true
PS > "357test*&(fdnsajkfj" -match "$pattern.*\w+$" # return true
PS > "357test*&(fdnsajkfj" -match "$pattern\w+$" # return false
Please have a try. :)
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