Question

The following code is a line in an xml file:

<appId>455360226</appId>

How can I replace the number between the 2 tags with another number using ruby?

Answer 1

There is no possibility to modify a file content in one step (at least none I know, when the file size would change).
You have to read the file and store the modified text in another file.

replace="100"
infile = "xmlfile_in"
outfile = "xmlfile_out"
File.open(outfile, 'w') do |out|
  out << File.open(infile).read.gsub(/<appId>\d+<\/appId>/, "<appId>#{replace}</appId>")
end  

Or you read the file content to memory and afterwords you overwrite the file with the modified content:

replace="100"
filename = "xmlfile_in"
outdata = File.read(filename).gsub(/<appId>\d+<\/appId>/, "<appId>#{replace}</appId>")

File.open(filename, 'w') do |out|
  out << outdata
end  

(Hope it works, the code is not tested)