Multiline Regex Lookbehind Failing in Powershell - regex

Question

I'm trying to parse a particular text file. One portion of the file is:
Installed HotFix
n/a Internet Explorer - 0
Applications:
In powershell, this is currently in a file C:\temp\software.txt. I'm trying to get it to return all lines in between "HotFix" and "Applications:" (As there may be more in the future.)
My current command looks like this:
Get-Content -Raw -Path 'C:\temp\software.txt' | Where-Object { $_ -match '(?<=HotFix\n)((.*?\n)+)(?=Applications)' }
Other regex I've tried:
'(?<=HotFix`n)((.*?`n)+)(?=Applications)'
'(?<=HotFix`n)((.*?\n)+)(?=Applications)'
'(?<=HotFix\n)((.*?`n)+)(?=Applications)'
'(?<=HotFix$)((.*?\n)+)(?=Applications)'
'(?<=HotFix)((.*?\n)+)(?=Applications)'
'(?<=HotFix)((.*?`n)+)(?=Applications)'

Answer 1

I think Select-String will provide better results here:
((Get-Content -Path 'C:\temp\software.txt' textfile -Raw |
Select-String -Pattern '(?sm)(?<=HotFix\s*$).*?(?=^Applications:)' -AllMatches).Matches.Value).Trim()
Regex modifier s is used because you are expecting the . character to potentially match newline characters. Regex modifier m is used so that end of string $ and start of string ^ characters can be matched on each line. Together that syntax is (?sm) in PowerShell.
Where {$_ -match ...} will return anything that makes the condition true. Since you are passing a Get-Content -Raw output, the entire contents of the file will be one string and therefore the entire string will output on a true condition.
Since you used -match here against a single string, any successful matches will be stored in the $matches automatic variable. Your matched string would be available in $matches[0]. If you were expecting multiple matches, -match will not work as constructed here.
Alternatively, the .NET Matches() method of the Regex class, can also do the job:
[regex]::Matches((Get-Content 'c:\temp\software.txt' -Raw),'(?sm)(?<=HotFix\s*$).*?(?=^Applications:)').Value.Trim()
Without Trim(), you'd need to understand your newline character situation:
[regex]::Matches((Get-Content software.txt -Raw),'(?m)(?<=HotFix\r?\n?)[^\r\n]+(?=\r?\n?^Applications:)').Value
A non-regex, alternative could use a switch statement.
switch -File Software.txt -Regex {
'HotFix\s*$' { $Hotfix,$Applications = $true,$false }
'^Applications:' { $Applications = $true }
default {
if ($Hotfix -and !$Applications) {
$_
}
}
}

Answer 2

If you read the file into a string the following regular expression will read the lines of interest:
/(?<=HotFix\n).*?(?=\nApplications:)/s
demo
The regex reads:
Match zero or more characters, lazily (?), preceded by the string "HotFix\n" and followed by the string "\nApplications:".
(?<=HotFix\n) is a positive lookbehind; (?=\nApplications:) is a positive lookahead.
The flag s (/s) causes .*? to continue past the ends of lines. (Some languages have a different flag that has the same effect.)
.*? (lazy match) is used in place of .* (greedy match) in the event that there is more than one line following the "Hot Fix" line that begins "Applications:". The lazy version will match the first; the greedy version, the last.
I would not be inclined to use a regex for this task. For one, the entire file must be read into a string, which could be problematic (memory-wise) if the file is sufficiently large. Instead, I would simply read the file line-by-line, keeping only the current line in memory. Once the "Bad Fix" line has been read, save the following lines until the "Applications:" line is read. Then, after closing the file, you're done.

Answer 3

Instead of using lookarounds, you could make use of a capturing group
First match the line that ends with HotFix. Then capture in group 1 all the following lines that do not start with Applications and then match Applications
^.*\bHotFix\r?\n((?:(?!Applications:).*\r?\n)+)Applications:
Explanation
^.*\bHotFix\r?\n Match the line that ends with HotFix
( Capture group 1
(?: Non capture group
(?!Applications:).*\r?\n Match the whole line if it does not start with Applications:
)+ Close non capturing group and repeat 1+ times to match all lines
) Close group 1
Applications: Match literally
Regex demo