How to Remove anything not letters or numbers from part of lines with Regular Expressions?

Question

How do I remove anything not letters or numbers from part of lines in txt file?

More explaining I have username:password or username;password.

I want to edit the username only and remove anything that not letters or numbers from it without touch the password. I would like do it with Regular Expressions since I am using Emeditor For Large Files and I believe Regular Expressions work in both Notepad++ and Emeditor.

What have I tried?
I did use Find: [^a-z0-9:;]+ but cant skip the password after : or ; so what I want to do is to skip line start from :password here and ;password here and only remove none letters or numbers from username alone.

Sorry if I didn't describe everything well, admins can edit if possible.

Full Example of lines:

!start._1:stop.~1@
Sta%rs&:B!ge(s+R}\
#Step[14,:St,./\Ert`
~user@#%name^*)+:P@$$wor'";D
T&*est~!@#$%^&*()_+={}|\;pass;word123
user@#%name;password!#$~`'123
45Star^5#$Lord1:@T1esting!
u~s#e%r^n&a*m(e)t_e+s-t,:Pa:ssw/orD$+;

Needed Result:

start1:stop.~1@
Stars:B!ge(s+R}\
Step:St,./\Ert`
username:P@$$wor'";D
Test;pass;word123
username;password!#$~`'123
45Star5Lord1:@T1esting!
usernametest:Pa:ssw/orD$+;

Answer 1

It can't be done in one pass.
Here is a way to do the job with multiple passes:

• Ctrl+H
• Find what: ^([^a-z0-9;:]*)([a-z0-9]*)(?1)(.*?[;:].+$)
• Replace with: $2$3
• Uncheck Match case
• check Wrap around
• check Regular expression
• DO NOT CHECK . matches newline
• Replace all (Click here as many times as needed, it replaces only a part of invalid characters each time)

Explanation:

^                   : begining of line
  (                 : start group 1
    [^a-z0-9;:]*    : negative character class, 0 or more any character that is not alpha-num or colon or semicolon
  )                 : end group 1
  (                 : start group 2
    [a-z0-9]*       : character class, 0 or more alpha-num
  )                 : end group 2
  (?1)              : re-use the pattern in group 1 (ie. [^a-z0-9;:]*)
  (                 : group 3
    .*?             : 0 or more any character but newline, not greedy
    [;:]            : a colon or semicolon (the first that exists in a line)
    .+              : 1 or more any character but newline (the password)
    $               : end of line
  )                 : end group 3

Replacement:

$2      : content of group 2, the alpha-num part of the name
$3      : content of group 3, rest of the line

Result for given example:

start1:stop.~1@
Stars:B!ge(s+R}\
Step14:St,./\Ert`               <== I guess there is a typo in your request
username:P@$$wor'";D
Test;pass;word123
username;password!#$~`'123
45Star5Lord1:@T1esting!
usernametest:Pa:ssw/orD$+;