How can I convert from underscores to camel case with a regex?

Question

How can I convert names with underscores into camel case names as follows using a single Java/Perl regular expression search and replace?

underscore_variable_name -> underscoreVariableName
UNDERSCORE_VARIABLE_NAME -> underscoreVariableName
_LEADING_UNDERSCORE -> leadingUnderscore

The reason I ask for a single regular expressions is that I want to do this using Eclipse or Notepad++ search and replace.

Answer 1

Some Perl examples:

my $str = 'variable_name, VARIABLE_NAME, _var_x_short,  __variable__name___';

### solution 1
$_ = $str;

$_ = lc;
s/_(\w)/\U$1/g;

say;

### solution 2: multi/leading underscore fix
$_ = $str;

$_ = lc;
s/(?<=[^\W_])_+([^\W_])|_+/\U$1/g;

say;

### solution 3: without prior lc
$_ = $str;

s/(?<=[^\W_])_+([^\W_])|([^\W_]+)|_+/\U$1\L$2/g;

say;

Output:

variableName, variableName, VarXShort,  _variable_name__
variableName, variableName, varXShort,  variableName
variableName, variableName, varXShort,  variableName

Answer 2

Uppercases letters following _-:

s/[_-]([a-z])/\u$1/gr

Answer 3

If you already have camelCase variables in the string, then @Qtax's answer will make them lowercase. If all of your variables are lower-case under_scored then you can make the following modification to #3: W --> A-Z

's/(?<=[^\A-Z_])_+([^\A-Z_])|([^\A-Z_]+)|_+/\U$1\L$2/g'