How does this Powershell Regex work?

Question

'<Relation From="701047080" ObjectNumber="166543300" Output="77" To="464616324">' -match '(?<=Output=")[^"]*'

$matchs then contains 77?

Which is what I want, I just don't understand why it works?

I've found stuff saying I could:

match '(?<NAME>Some regex)'

But I have no Idea what '(?<=' does? And I would REALLY like to understand the syntax...

I actually need to pull out the value of Output and To both... And only have powershell on the system I need to do this on...

Answer 1

The expression (?<=Output=") is a lookbehind . It matches only immediately after the string Output=" .

The expression [^"]* is a negated character class . It matches all characters until the next double-quote (or to the end of the string if there are no more double-quotes).

Answer 2

The [^"]* says any number of characters other than " .

The [^"]* is the main expression.

The (?<=Output=") is a lookbehind and makes sure that the main expression is following the expression in the lookbehind, in this case Output=" . The lookbehind expression is not included in the match

I don't think the regex is optimal.

You could have used:

'<Relation From="701047080" ObjectNumber="166543300" Output="77" To="464616324">' -match 'Output="(.*?)"'

(with the regex being as simple as Output="(.*?)" )

and got the 77 in $matches[1]

The regex was probably just complicated with the lookbehind to make the Output=" be not part of the match. It makes the regex more complex and reduces performance. All you need is the above regex and extract 77 with the appropriate group.