Use a function in Powershell replace - regex

Question

I'm trying to replace part of a string in Powershell. However, the replacement string is not hardcoded, it's calculated from a function:
$text = "the image is -12345-"
$text = $text -replace "-(\d*)-", 'This is the image: $1'
Write-Host $text
This gives me the correct result:
"This is the image: 12345"
Now, I want to include the base64 encoded image. I can read the image from the id. I was hoping the following would work, but it doesn't:
function Get-Base64($path)
{
[convert]::ToBase64String((get-content $path -encoding byte))
}
$text -replace "-(\d*)-", "This is the image: $(Get-Base64 '$1')"
The reason that it doesn't work, is because it first passes $1 (the string, not the value of $1) to the function, executes it and only then does it do the replace. What I want to do is
Find the occurrence of the pattern
Replace each occurence with the pattern
For each replace:
Pass the capture group to the function
Use the value of the capture group to get the base64 image
inject the base64 image into the replacement

Answer 1

You can use the static Replace method from the [regex] class:
[regex]::Replace($text,'-(\d*)-',{param($match) "This is the image: $(Get-Base64 $match.Groups[1].Value)"})
Alternatively you can define a regex object and use the Replace method of that object:
$re = [regex]'-(\d*)-'
$re.Replace($text, {param($match) "This is the image: $(Get-Base64 $match.Groups[1].Value)"})
For better readability you could define the callback function (the scriptblock) in a separate variable and use that in the replacement:
$callback = {
param($match)
'This is the image: ' + (Get-Base64 $match.Groups[1].Value)
}
$re = [regex]'-(\d*)-'
$re.Replace($text, $callback)

Answer 2

PetSerAl's helpful answer is your only option in Windows PowerShell, as of v5.1.
PowerShell Core v6.1+ now offers a native PowerShell solution via an enhancement to the
-replace operator, which obviates the need to call [regex]::Replace():
Just as with [regex]::Replace(), you can now:
pass a script block as the -replace replacement operand, which must return the replacement string,
except that the match at hand (an instance of type [System.Text.RegularExpressions.Match]) is represented as automatic variable $_, as is customary in PowerShell.
Applied to your case:
$text -replace "-(\d*)-", { "This is the image: $(Get-Base64 $_.Groups[1].Value)" }
A simpler example:
# Increment the number embedded in a string:
PS> '42 years old' -replace '\d+', { [int] $_.Value + 1 }
43 years old

Answer 3

Here's another way. Use the -match operator, and then reference $matches. Note that $matches doesn't get set with arrays on the left side of the -match operator. $matches.1 is the first grouping formed by the ( ).
$text = "the image is -12345-"
function Get-Base64($path) {
[convert]::ToBase64String( (get-content $path -asbytestream) ) } # ps 6 ver
if (! (test-path 12345)) { echo hi > 12345 }
$text -match '-(\d*)-'
$text -replace '-(\d*)-', "$(Get-Base64 $matches.1)"
the image is aGkNCg==
Or to break it up even more:
$text -match '-(\d*)-'
$result = Get-Base64 $matches.1
$text -replace '-(\d*)-', $result