Is equivalent to [A-Za-z0-9]?

Ruby
1

Hi,

I’m trying to creating a regex that will match a string that starts and
ends with an alphanumeric character, and has alphanumeric characters or
dashes in the middle.

This regex gives the expected result:
irb(main):008:0> /^[:alnum:][A-Za-z0-9-][A-Za-z0-9]$/ =~ “apple!”
=> nil
irb(main):009:0> /^[:alnum:][A-Za-z0-9-][A-Za-z0-9]$/ =~ “apple”
=> 0

This regex works as expected on “apple!”, but returns nil on “apple”:
irb(main):010:0> /^[:alnum:][A-Za-z0-9-][:alnum:]$/ =~ “apple!”
=> nil
irb(main):011:0> /^[:alnum:][A-Za-z0-9-][:alnum:]$/ =~ “apple”
=> nil

Can anyone explain this?

Thanks,
Jeff

2

Jeff wrote:

=> 0

This regex works as expected on “apple!”, but returns nil on “apple”:
irb(main):010:0> /^[:alnum:][A-Za-z0-9-][:alnum:]$/ =~ “apple!”
=> nil
irb(main):011:0> /^[:alnum:][A-Za-z0-9-][:alnum:]$/ =~ “apple”
=> nil

Can anyone explain this?

Yes, that’s a classical mistake: POSIX specifications of this kind
must be included inside other []. Try rather

/^[[:alnum:]][A-Za-z0-9-]*[[:alnum:]]$/

The way you wrote it just meant ‘:’ or ‘a’ or ‘l’ or…

Vince

3

/^[[:alnum:]][A-Za-z0-9-]*[[:alnum:]]$/

Incidentally, the dash doesn’t need to be escaped in the character class
if you put it first:

/^[[:alnum:]][-A-Za-z0-9]*[[:alnum:]]$/

best,
Dan

4

VIncent and Daniel,

Thanks very much.

– Jeff