regex - and . _ - regex

Question

Writing a regex for a-z, A-Z and allowing - and . _ and integers.
For example:
Testing-Server1
Testing.Server
Testing_Server
Tried this, but was unsure how to allow - _ . and integers:
"^[a-z][A-Z]*$"

Answer 1

Simple enough:
^[-a-zA-Z0-9_.]*$
Explanation
^ = Match from the start of the input
[-a-zA-Z0-9_.] = A character class (a list of allowed characters):
- matches the literal '-' character (must be the first or last character in the class)
a-z matches lowercase alpha characters
A-Z matches uppercase alpha characters
0-9 matches the numeric characters
_ matches the literal '_' character
. matches the literal '.' character (unlike outside a character class, where it matches any character)
* = Match 0 to infinite characters (use + to match at least one character)
$ = Match to the end of the string
Alternative
As stranac mentions in his answer, you can replace a-zA-Z0-9_ with \w, but I prefer the more explicit version, as it's more understandable.
Limiting matched characters
As the OP asked in a comment, to limit the allowed number of characters to 15:
^[-a-zA-Z0-9_.]{0,15}$
Where {0,15} means match between 0 and 15 characters (of the character class) only. You can adjust the values as appropriate, for example, to match at least one character, use {1,15}.

Answer 2

The other answers are over-complicating things.
\w already matches letters, digits and underscores, so you only need to add dot and minus to those.
This regex does the trick: r'^[\w.-]+$'
A few examples:
>>> re.search(r'^[\w.-]+$', 'Testing-Server1').group()
'Testing-Server1'
>>> re.search(r'^[\w.-]+$', 'Testing.Server').group()
'Testing.Server'
>>> re.search(r'^[\w.-]+$', 'Testing_Server').group()
'Testing_Server'

Answer 3

Instead of parsing all the string to check if it contains only allowed characters, it is faster to search the first character that is not an allowed character, because the search stops once it is found:
if not re.search(r'[^\w.-]', yourstring):
...
If you need to check the max length of the string, you can simply write:
if (len(yourstring) < 16 and not re.search(r'[^\w.-]', yourstring)):

Answer 4

This following pattern is enough
^[a-zA-z1-9._-]+$
Explanation
a-z a single character in the range between a and z (case sensitive)
A-z a single character in the range between A and z (case sensitive)
1-9 a single character in the range between 1 and 9
. matches the character . literally
the literal character -
Demo

Answer 5

You can try the following pattern :
[a-zA-Z\d._-]+
DEMO

Answer 6

Try this out :
([a-zA-z0-9._-]+)
Explanation:
1) a-z a single character in the range between a and z (case sensitive)
2) A-z a single character in the range between A and z (case sensitive)
3) 0-9 a single character in the range between 0 and 9
4) ._- a single character in the list ._- literally
It should work , check this :https://regex101.com/r/gZ5xN5/2