let: string str =
"a=<random text>
a=pattern:<random text (may be fixed length)>
a=<random text>";
Suppose, from above string a= and pattern are fixed. There may or may not be \n after each line.
How to remove a=pattern:<random> from the str string?
You could use regex capturing groups. Capture the text before and after the text you want to remove.
let newStr = str.replace(/(a=.+(\n)?)(a=pattern:.+)((\n)?a=.+)/, "$1$4")