sed -r 's/([a-z]+)([A-Z][a-z]+)/\1_\l\2/g' file.txt

Convert camelCase to underscores (camel_case)

Useful for switching over someone else's coding style who uses camelCase notation to your style using all lowercase with underscores.
Sample Output
$ cat file.txt
camelCase
$ sed -r 's/([a-z]+)([A-Z][a-z]+)/\1_\l\2/g' file.txt
camel_case

11
By: atoponce
2009-04-28 22:44:45
sed

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What Others Think

A great one. Thanks. You missed the underscore though, this should be: sed -r 's/([a-z]+)([A-Z][a-z]+)/\1_\l\2/g' file.txt
AmirWatad · 473 weeks and 2 days ago
Fixed. I had it in the sample output, but must have missed it in the command itself. Thanks.
atoponce · 473 weeks and 2 days ago
Good job
kaedenn · 473 weeks and 1 day ago
Where's the reverse? :-)
furicle · 473 weeks and 1 day ago
btw, it's good for names like camelCase, but not for camelCaseLong :)
AmirWatad · 473 weeks and 1 day ago
I think this is a little more robust: It converts CamelCaseWord or camelCaseWord to camel_case_word (the last pipe is needed to handle the CamelCaseWord case) sed 's/\([A-Z]\)/_\l\1/g' file.txt | sed 's/^_\([a-z]\)/\1/g'
AmirWatad · 473 weeks and 1 day ago
and this is the reverse: camel_case_word to camelCaseWord: sed 's/_\([a-z]\)/\u\1/g' file.txt camel_case_word to CamelCaseWord sed 's/_\([a-z]\)/\u\1/g' file.txt | sed 's/^\([a-z]\)/\u\1/g'
AmirWatad · 473 weeks and 1 day ago
LC_COLLATE=C LC_CTYPE=C export LC_CTYPE LC_COLLATE The exports above fix a problem where [a-z] is case-insensitive Here's an explanation from the sed man page (Gnu Sed 4) [a-z] is case insensitive You are encountering problems with locales. POSIX mandates that [a-z] uses the current locale's collation order - in C parlance, that means using strcoll(3) instead of strcmp(3). Some locales have a case-insensitive collation order, others don't. Another problem is that [a-z] tries to use collation symbols. This only happens if you are on the GNU system, using GNU libc's regular expression matcher instead of compiling the one supplied with GNU sed. In a Danish locale, for example, the regular expression ^[a-z]$ matches the string `aa', because this is a single collating symbol that comes after `a' and before `b'; `ll' behaves similarly in Spanish locales, or `ij' in Dutch locales. To work around these problems, which may cause bugs in shell scripts, set the LC_COLLATE and LC_CTYPE environment variables to `C'. Here's an example of a line that was having problems due to the case-insensitive problem. Exporting LC_COLLATE and LC_CTYPES fixed the problem. It has to be done each time you log in though. I'm hesitant to put this export into my .profile, as I'm not sure what it will do to the rest of the system programs. sed -r 's/([a-z]+)([A-Z][a-z]+)/\1_\l\2/g' invalid_name2.txt
Christian_Long · 413 weeks ago
sed -r 's/([^A-Z-])([A-Z])/\1_\2/g' file.txt Replace CamelCaseWord by Camel_Case_Word
franek · 395 weeks and 6 days ago

What do you think?

Any thoughts on this command? Does it work on your machine? Can you do the same thing with only 14 characters?

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