How to replace all occurrences of a word with another word in all files in multiple directories and their sub-directories using bash?

I have a need to replace all email addresses of form admin@xxxx.com or admin@yyy.net or admin@zzzzz.org with root@localhost in all files under several folders and all their sub-folders. How can I do it with bash programming?

find dir1 dir2 dir3 dir4 -type f -exec sed -i 's/admin@.\{1,27\}\.\(com\|net\|org\)/root@localhost/g' {} \;

The -i option tells sed to do in-place replacement. The regexp in sed is complex which I will explain in details.

The regular expression(pattern) in sed is put in //. The dot symbol in regexp has special meaning, i.e., denoting any character. So to match the dot character itself, you should escape it with \.

Interestingly, the characters {,} and (,),| which have special meanings do not need to be escaped if you want their literal meanings. Instead, you need to escape them with \ to obtain their special meanings. If you do not escape them with \, they are just matched against {,}, (,),|.

More interestingly, the character [-] which have special meanings do not need to be escaped to use their special meanings. However, you need to escape them if you want their literal meanings. The -, if not escaped, has its literal meaning when not in [] context, but has its regexp meaning when in [] context. The -, if escaped, still has its regexp meaning when in [] and has literal meaning when not in [] context.

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