REGEX .net\powershell match string between strings

Use look arounds:

(?<=MESSAGE BEGIN)[\s\S]+(?=MESSAGE END)

Will match any text between (but not including) MESSAGE BEGIN and MESSAGE END.

For discussion of supported regular expresions in Powershell visit: https://blogs.technet.microsoft.com/heyscriptingguy/2016/10/21/powershell-regex-crash-course-part-4-of-5/

The first part here is to use a pattern like [\s\S]* instead of the . to match newlines too. You want to match lazy+?/*? to avoid to match too much (e.g. from the first MESSAGE BEGIN to the last MESSAGE END if there are multiple message blocks.)

Pattern:

MESSAGE BEGIN([\s\S]*?)MESSAGE END

or if you just want the inner part use look-arounds (still lazy *?):

(?<=MESSAGE BEGIN)[\s\S]*?(?=MESSAGE END)

End-to-end code sample:

$text = [IO.File]::ReadAllText(".\a.txt")

$matches = [regex]::matches($text, "MESSAGE BEGIN([\s\S]*?)MESSAGE END");
ForEach($match in $matches) {
  #Write-Output $match.Value.Trim(); #if you use look-arounds
  Write-Output $match.Groups[1].Value.Trim();
}

MESSAGE BEGIN(\s|\S)*MESSAGE END

(.*?) matches all characters, except for line terminators.

\s matches any whitespace character (equal to [\r\n\t\f\v ])

\S matches any non-whitespace character (equal to [^\r\n\t\f\v ])

Include a bar | in the capture group to match either \s or \S

Then a star * after the capture group to match zero to unlimited characters

Link to example

I've created an example in javascript.

const texto = `
MESSAGE BEGIN

mary had a little lamb.

little lamb

MESSAGE END
`

const regex = /MESSAGE\sBEGIN[\s\S]*MESSAGE\sEND/gi

console.log(texto.match(regex))

The output is:
[ 'MESSAGE BEGIN\n\nmary had a little lamb.\n\nlittle lamb\n\nMESSAGE END' ]

The breaklines were kept.