If you open the original file in Notepad++, and enable View > Show Symbol > Show All Characters, you should see little black boxes for the end of line, either two with CR and LF, or one or the other… Plus, your status bar should list what EOL Notepad++ detects (Windows (CR LF), Unix (LF), or Macintosh (CR)). If you give us a screenshot (see the help-with-markdown post for screenshot help; use the ![](url.to/image.png)-notation so the image is visible in the forum), we might be able to see why the regex didn’t work in your original file.

To rule out the possibility that the data file (.xml) is corrupt, I copied a few
lines (one by one) to the plain (windows) text file, and then had Notepad++
load that one, but saw the same error…

I’m on Windows 10 with Notepad++ v7.6.2 in case this matters.

name=john
numb=053a82bf31d0f63e669e9035bea4f311
name=pete
numb=a0c49c5e34724c1247b314a3647c3acb
name=sam
name=eric
numb=e368627d9bd60c6e38349532d966df4e
name=steve
numb=b03438713f2894d97f139721f98b2794

• Find What = (?-s)(name=.*)\R(numb=.*)$

• Replace With = $1 : $2

• Mode = Regular Expression

• the first term tells it to have . not match EOL characters

• the second puts the name=somethingerother line into $1

• the \R eats a newline (EOL) sequence

• the final parenthetical stores the numb=... into $2

• the $ says it must grab until the end-of-line

in the replace, it’s just the two stored values, separated by a literal space, colon, space. (You wanted ideas, and didn’t specify a separator)

result =

name=john : numb=053a82bf31d0f63e669e9035bea4f311
name=pete : numb=a0c49c5e34724c1247b314a3647c3acb
name=sam
name=eric : numb=e368627d9bd60c6e38349532d966df4e
name=steve : numb=b03438713f2894d97f139721f98b2794

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FYI: if you have further regex needs, study this FAQ and the documentation it points to. Before asking a new regex question, understand that for future requests, many of us will expect you to show what data you have (exactly), what data you want (exactly), what regex you already tried (to show that you’re showing effort), why you thought that regex would work (to prove it wasn’t just something randomly typed), and what data you’re getting with an explanation of why that result is wrong. When you show that effort, you’ll see us bend over backward to get things working for you. If you need help formatting the data so that the forum doesn’t mangle it (so that it shows “exactly”, as I said earlier), see this help-with-markdown post, where @Scott-Sumner gives a great summary of how to use Markdown for this forum’s needs.
Please note that for all “regex” queries – or queries where you want help “matching” or “marking” or “bookmarking” a certain pattern, which amounts to the same thing – it is best if you are explicit about what needs to match, and what shouldn’t match, and have multiple examples of both in your example dataset. Often, what shouldn’t match helps define the regular expression as much or more than what should match.