capture all characters between match character (single or repeated) on string

I'm trying to extract the string preceding a specific character (even when character is repeated, like this (ie: underscore '_'):

this_is_my_example_line_0
this_is_my_example_line_1_
this_is_my_example_line_2___
_this_is_my_ _example_line_3_
__this_is_my___example_line_4__

and after running my regex I should get this (the regex should ignore the any instances of the matching character in the middle of the string):

this_is_my_example_line_0
this_is_my_example_line_1
this_is_my_example_line_2
this_is_my_ _example_line_3
this_is_my___example_line_4

In other words I'm trying to 'trim' the matched character(s) at the beginning and end of string.

I'm trying to use a Regex in Java to accomplish this, my idea is to capture the group of characters between the special character(s) at the end or beginning of the line.

So far I can only do this successfully for example 3 with this regexp:

/[^_]+|_+(.*)[_$]+|_$+/

[^_]+ not 'underscore' once or more 
| OR 
_+ underscore once or more
(.*) capture all characters
[_$]+ not 'underscore' once or more followed by end of line
 |_$+ OR 'underscore' once or more followed by end of line

I just realized that this excludes the first word of the message on example 0,1,2 since the string doesn't start with underscore and it only starts matching after finding a underscore..

Is there an easier way not involving regex? I don't really care about the first character (although it would be nice) I only need to ignore the repeating character at the end.. it looks that (by this regex tester) just doing this, would work? /()_+$/ the empty parenthesis matches anything before a single or repeting matches at the end of the line.. would that be correct?

Thank you!

Solution

There are a couple of options here, you could either replace matches of ^_+|_+$ with an empty string, or extract the contents of the first capture group from the match of ^_*(.*?)_*$. Note that if your strings may be multiple lines and you want to perform the replacement on each line then you will need to use the Pattern.MULTILINE flag for either approach. If your strings may be multiple lines and you only want to replacement to occur at the very beginning and end, don't use Pattern.MULTILINE but use Pattern.DOTALL for the second approach.

For example: http://regexr.com?355ff