Powershell regex match sequence doesn't work although it matches in Sublime Text find and replace - regex

Question

I am trying to create a Powershell regex statement to remove the top five lines of this output from a git diff file that has already been modified with Powershell regex.
[1mdiff --git a/uk1.adoc b/uk2.adoc</span>+++
[1mindex b5d3bf7..90299b8 100644</span>+++
[1m--- a/uk1.adoc</span>+++
[1m+++ b/uk2.adoc</span>+++
[36m## -1,9 +1,9 ##</span>+++
= Heading
Body text
Image shown because binary code doesn't show in the text
The following statement matches the text so the '= Heading' line is placed at the top of the page if I replace with nothing.
^[^=]*.[+][\n]
But in Powershell, it isn't matching the text.
Get-Content "result2.adoc" | % { $_ -Replace '^[^=]*.[+][\n]', '' } | Out-File "result3.adoc";
Any ideas about why it doesn't work in Powershell?
My overall goal is to create a diff file of two versions of an AsciiDoc file and then replace the ASCII codes with HTML/CSS code to display the resulting AsciiDoc file with green/red track changes.

Answer 1

The simplest - and faster - approach is to read the input file as a single, multiline string with Get-Content -Raw and let the regex passed to -replace operate across multiple lines:
(Get-Content -Raw result2.adoc) -replace '(?s)^.+?\n(?==)' |
Set-Content result3.adoc
(?s) activates in-line option s which makes . match newline (\n) characters too.
^.+?\n(?==) matches from the start of the string (^) any number of characters (including newlines) (.+), non-greedily (?)
until a newline (\n) followed by a = is found.
(?=...) is a look-ahead assertion, which matches = without consuming it, i.e., without considering it part of the substring that matched.
Since no replacement operand is passed to -replace, the entire match is replace with the implied empty string, i.e., what was matched is effectively removed.
As for what you tried:
The -replace operator passes its LHS through if no match is found, so you cannot use it to filter out non-matching lines.
Even if you match an undesired line in full and replace it with '' (the empty string), it will show up as an empty line in the output when sent to Set-Content or Out-File (>).
As for your specific regex, ^[^=]*.[+][\n] (whether or not the first ^ is followed by an ESC (0x1b) char.):
[\n] (just \n would suffice) tries to match a newline char. after a literal + ([+]), yet lines read individually with Get-Content (without -Raw) by definition are stripped of their trailing newline, so the \n will never match; instead, use $ to match the end of a line.
Instead of % (the built-in alias for the ForEach-Object cmdlet) you could have used ? (the built-in alias for the Where-Object cmdlet) to perform the desired filtering:
Get-Content result2.adoc | ? { $_ -notmatch '^\e\[' }
$_ -notmatch '^\e[' returns $True only for lines that don't start (^) with an ESC character (\e, whose code point is 0x1b) followed by a literal (\) [, thereby effectively filtering out the lines before the = Heading line.
However, the multi-line -replace command at the top is a more direct and faster expression of your intent.

Answer 2

Here is the code I ended up with after help from #mklement0. This Powershell script creates MS Word-style track changes for two versions of an AsciiDoc file. It creates the Diff file, uses regex to replace ASCII codes with HTML/CSS tags, removes the Diff header (thank you!), uses AsciiDoctor to create an HTML file and then PrinceXML to create a PDF file of the output that I can send to document reviewers.
git diff --color-words file1.adoc file2.adoc > result.adoc;
Get-Content "result.adoc" | % {
$_ -Replace '(=+ ?)([A-Za-z\s]+)(\[m)', '$1$2' `
-Replace '\[32m', '+++<span style="color: #00cd00;">' `
-Replace '\[31m', '+++<span style="color: #cd0000; text-decoration: line-through;">' `
-Replace '\[m', '</span>+++' } | Out-File -encoding utf8 "result2.adoc" ;
(Get-Content -Raw result2.adoc) -replace '(?s)^.+?\n(?==)', '' | Out-File -encoding utf8 "result3.adoc" ;
asciidoctor result3.adoc -o result3.html;
prince result3.html --javascript -o result3.pdf;
Read-Host -Prompt "Press Enter to exit"
Here's a screenshot of the result using some text from Wikipedia: