text - Using sed to change only consecutive repeated letters

Question

Using sed, how to change the letter 'a' to 'A' but only if it appears repeated as two or more consecutive letters. Example, from:

galaxy
ear
aardvak
Haaaaaaaaa 

into

galaxy
ear
AArdvak
HAAAAAAAAA 

Answer 1

You can do it using groups. If you have this file:

$ cat a
galaxy
ear
aardvak
Haaaaaaaaa 
Ulaanbaatar

You can use this sed command:

$ sed 's/(.)1{1,}/U&/g' a
galaxy
ear
AArdvak
HAAAAAAAAA 
UlAAnbAAtar

What does happen here? If we have a char, "packed" in a group ((.)), and this group (1) repeats itself one or more times (1{1,}), then replace the matched part (&) by its uppercased version (U&).