grep gets a regex and one or more files. It goes over line-by-line all the files and displays the lines where the regex matched. A few examples:
grep python file.xml # lines that have the string python in them in file.xml. grep  file.xml # lines that have either 3 or 4 (or both) in file.xml. grep  *.xml # lines that have either 3 or 4 (or both) in every xml file. grep [0-9] *.xml # lines with a digit in them. egrep '\b[0-9]' *.xml # only highlight digits that are at the beginning of a number.