CLI tip 33: manipulating string case with GNU sed

sed provides escape sequences to change the case of replacement strings, which might include backreferences, shell variables, etc.

Sequence	Description
`\E`	indicates the end of case conversion
`\l`	convert the next character to lowercase
`\u`	convert the next character to uppercase
`\L`	convert the following characters to lowercase (overridden by `\U` or `\E`)
`\U`	convert the following characters to uppercase (overridden by `\L` or `\E`)

First up, changing case of only the immediate next character after the escape sequence.

# match only the first character of a word
# use & to backreference the matched character
# \u would then change it to uppercase
$ echo 'hello there. how are you?' | sed 's/\b\w/\u&/g'
Hello There. How Are You?

# change the first character of a word to lowercase
$ echo 'HELLO THERE. HOW ARE YOU?' | sed 's/\b\w/\l&/g'
hELLO tHERE. hOW aRE yOU?

# match lowercase followed by underscore followed by lowercase
# delete the underscore and convert the 2nd lowercase to uppercase
$ echo '_fig aug_price next_line' | sed -E 's/([a-z])_([a-z])/\1\u\2/g'
_fig augPrice nextLine

Next, changing case of multiple characters at a time.

# change all alphabets to lowercase
$ echo 'HaVE a nICe dAy' | sed 's/.*/\L&/'
have a nice day
# change all alphabets to uppercase
$ echo 'HaVE a nICe dAy' | sed 's/.*/\U&/'
HAVE A NICE DAY

# \E will stop further conversion
$ echo 'fig_ aug_price next_line' | sed -E 's/([a-z]+)(_[a-z]+)/\U\1\E\2/g'
fig_ AUG_price NEXT_line
# \L or \U will override any existing conversion
$ echo 'HeLLo:bYe gOoD:beTTEr' | sed -E 's/([a-z]+)(:[a-z]+)/\L\1\U\2/Ig'
hello:BYE good:BETTER

Finally, examples where escapes are used next to each other.

# uppercase first character of a word
# and lowercase rest of the word characters
# note the order of escapes used, \u\L won't work
$ echo 'HeLLo:bYe gOoD:beTTEr' | sed -E 's/[a-z]+/\L\u&/Ig'
Hello:Bye Good:Better

# lowercase first character of a word
# and uppercase rest of the word characters
$ echo 'HeLLo:bYe gOoD:beTTEr' | sed -E 's/[a-z]+/\U\l&/Ig'
hELLO:bYE gOOD:bETTER

Video demo:

info See also my CLI text processing with GNU sed ebook.