Regex match character before and after underscore

Question

I have to write a regex with matches following:

• String should start with alphabets - [a-zA-Z]
• String can contain alphabets, spaces, numbers, _ and - (underscore and hyphen)
• String should not end with _ or - (underscore and hyphen)
• Underscore character should not have space before and after.

I came up with the following regex, but it doesn't seems to work

/^[a-zA-Z0-9]+(\b_|_\b)[a-zA-Z0-9]+$/

Test case:

HelloWorld // Match
Hello_World //Match
Hello _World // doesn't match
Hello_ World // doesn't match
Hello _ World // doesn't match
Hello_World_1 // Match
He110_W0rld // Match
Hello - World // Match
Hello-World // Match
_HelloWorld // doesn't match
Hello_-_World // match

Answer 1

You may use

^(?!.*(?:[_-]$|_ | _))[a-zA-Z][\w -]*$

See the regex demo

Explanation :

• ^ - start of string
• (?!.*(?:[_-]$|_ | _)) - after some chars ( .* ) there must not appear ( (?!...) ) a _ or - at the end of string ( [_-]$ ), nor space+ _ or _ +space
• [a-zA-Z] - the first char matched and consumed must be an ASCII letter
• [\\w -]* - 0+ word ( \\w = [a-zA-Z0-9_] ) chars or space or -
• $ - end of string

Answer 2

You could use this one:

^(?!^[ _-]|.*[ _-]$|.* _|.*_ )[\w -]*$

regex tester

For the test cases I used modifier gm to match each line individually.

If emtpy string should not be considered as acceptable, then change the final * to a + :

^(?!^[ _-]|.*[ _-]$|.* _|.*_ )[\w -]+$

Meaning of each part

• ^ and $ match the beginning/ending of the input
• (?! ) : list of things that should not match:
  • | : logical OR
  • ^[ _-] : starts with any of these three characters
  • .*[ _-]$ : ends with any of these three characters
  • .* _ : has space followed by underscore anywhere
  • .*_ : has underscore followed by space anywhere
• [\\w -] : any alphanumeric character or underscore (also matched by \\w ) or space or hyphen
• * : zero or more times
• + : one or more times

Answer 3

What about this?

^[a-zA-Z](\B_\B|[a-zA-Z0-9 -])*[a-zA-Z0-9 ]$

Broken down:

^               
[a-zA-Z]        allowed characters at beginning
(
 \B_\B          underscore with no word-boundary
|                 or
 [a-zA-Z0-9 -]  other allowed characters
)*
[a-zA-Z0-9 ]    allowed characters at end
$

Answer 4

Oh! I love me some regex!

Would this work? /^[az]$|^[az](?:_(?=[^ ]))?(?:[az\\d -][^ ]_[^ ])*[az\\d -]*[^_-]$/i

I was a tad unsure of rule 4--do you mean underscores can have a space before or after or neither, but not before and after?