Regex: Any word and numeric combination but with a single underscore

Question

I would like to match a whole string if it contains letters or numbers or a single underscore sequence, so:

Accepted:

• H_i_there
• _this_is_OK_
• _x_y_5_z
• abddd_cdxxx

Not accepted:

• s___2
• __s__t__2
• __x____x4

So multiple, consecutive underscores are not allowed. How does the regex expression look for this? My version is: ^[a-zA-Z0-9]+\_{0,1}[a-zA-Z0-9]+\_{0,1}$, but this must be recursive, somehow, for it to work, and AFAIK, regex does not support such complicated functionality.

Answer 1

You may use

^_?[a-zA-Z0-9]+(?:_[a-zA-Z0-9]+)*_?$

See the regex demo

Details:

• ^ - start of string
• _? - an optional _
• [a-zA-Z0-9]+ - 1+ alphanumeric chars
• (?:_[a-zA-Z0-9]+)* - 0+ sequences of:
  • _ - 1 underscore
  • [a-zA-Z0-9]+ - 1+ alphanumeric chars
• _? - an optional _
• $ - end of string

A less efficient, but shorter pattern:

^(?:_?[a-zA-Z0-9]+)*_?$

See this demo.