how to grep a word with only one single capital letter?

Question

The txt file is :

bar
quux
kabe
Ass
sBo
CcdD
FGH

I would like to grep the words with only one capital letter in this example, but when I use "grep [A-Z]", it shows me all words with capital letters.

Could anyone find the "grep" solution here? My expected output is

Ass
sBo

Does "word" mean letters only? By default, "word" characters are letters, digit and the underscore. Please clarify - all your test inputs are all-letters, but is that true in your real data? — , Apr 04 '20 at 17:31
@oguzismail - is that a `grep` solution, as the OP requested? — , Apr 04 '20 at 17:37
@mathguy Maybe the OP does not know other tools that could also be useful (not improbable for newcomers). And it's a standard utility, so why not mention it? — Quasímodo, Apr 04 '20 at 17:59

score 1 · Answer 1 · answered Apr 04 '20 at 17:44

1

grep '\<[a-z]*[A-Z][a-z]*\>' my.txt

will match lines in the ASCII text file my.txt if they contain at least one word consisting entirely of ASCII letters, exactly one of which is upper case.

answered Apr 04 '20 at 17:44

score 0 · Accepted Answer · answered Apr 04 '20 at 18:20

You seem to have a text file with each word on its own line.

You may use

grep '^[[:lower:]]*[[:upper:]][[:lower:]]*$' file

See the grep online demo.

The ^ matches the start of string (here, line since grep operates on a line by lin basis by default), then [[:lower:]]* matches 0 or more lowercase letters, then an [[:upper:]] pattern matches any uppercase letter, and then [[:lower:]]* matches 0+ lowercase letters and $ asserts the position at the end of string.

If you need to match a whole line with exactly one uppercase letter you may use

grep '^[^[:upper:]]*[[:upper:]][^[:upper:]]*$' file

The only difference from the pattern above is the [^[:upper:]] bracket expression that matches any char but an uppercase letter. See another grep online demo.

To extract words with a single capital letter inside them you may use word boundaries, as shown in mathguy's answer. With GNU grep, you may also use

grep -o '\b[^[:upper:]]*[[:upper:]][^[:upper:]]*\b' file
grep -o '\b[[:lower:]]*[[:upper:]][[:lower:]]*\b' file

See yet another grep online demo.

how to grep a word with only one single capital letter?

2 Answers2