regex101: PCRE2 (PHP) Substitution Workspace

^start of line

Negative Lookbehind(?<![\s\S\r])

Character Class[\s\S\r]

\sany whitespace character

\Sany non-whitespace character

\ra carriage return (ASCII 13)

Group 1(.*)

.*any character (except for line terminators), repeated any number of times

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

m modifier: multiline. Causes ^ and $ to match the start and end of each line, not only the start and end of the string

Search reference

Regular Expression

^(?<![\s\S\r])(.*)

New

# project1
I could have put any other inline tag or string there.
This main body line could have been the very first line as well, if no tags exist.

Change the keys after each `\n` and add or remove any more keys you want.
In this example, I added the desired tag in the indented multi-line format favoured by Obsidian.
As this is not a Templater solution, `date modified` is hard-coded here for the date you are making your changes. If not needed, it can be removed for the substitution part.  
If it is a first time doing regex changes, make a backup of vault or Diary folder.

Regular Expressions 101

Regular Expression

Extraction