Vim note

1.  ：g/pattern/s/old/new

The first g tells the command to operate on all lines of a file. pattern identifies the lines
on which a substitution is to take place. On those lines containing pattern, ex is to
substitute (s) for old the characters

old could be regular expression

2.  :%s/old/new/g

% indentifies all lines

s substitudes command

"old" could be regular expression