Why does Powershell string match escaped regex return false but still output correct match?-CodePudding

The code below works as is. The commented out lines are the alternative version which does not work.

[string]$secretString = "Secret message: The agent
was a man called Bond
007 BOND, James MI5 London
....... end"

# [string]$agent = '007 BOND'   # [regex]::Escape($agent)
[string]$result = $null

$secretString -match "(007 BOND[^\r\n]*)"
# $secretString -match "([regex]::Escape($agent)[^\r\n]*)"
$result = $Matches[0]

Write-Host "The full name of agent is: $result"

The output for working version is:

True
The full name of agent is: 007 BOND, James MI5 London

The output for non-working version (uncomment $agent declaration & swap $secretString -match line from explicit to escaped) is:

False
The full name of agent is: 007 BOND, James MI5 London

How can this happen? As I understand from Matching operators the $Matches automatic variable should be overwritten.

As it happens. If I open a new terminal in VS, to run the errant code again, I now get:

False
InvalidOperation:
    15 |  $result = $Matches[0]
         |  ~~~~~~~~~~~~~~~~~~~~~
         | Cannot index into a null array.

The messaging & logic is now consistent. This means I have two problems.

What is wrong with my regex escape code?
Why is the $Matches automatic variable not being overwritten? On the second run of the code it should be overwritten to null because the comparison operator match returns a false match.

Any suggestions would be appreciated.

CodePudding user response：

As for the second point, check the note section here regarding matches:

When $Matches is populated in a session, it retains the matched value until it's overwritten by another match. If -match is used again and no match is found, it doesn't reset $Matches to $null. The previously matched value is kept in $Matches until another match is found.