Regexp help - only want a portion of a entity

UG,

We have auto numbering on our accounts. I am trying to use a regular expression to only retrieve the ID portion of Name/ID. I have a formula field where I've attempted to use LTRIM or RTRIM, and even REGEXP_SUBSTR to isolate the portion of the string that I want, but I'm running into troubles.

I've see the following out there: rtrim(regexp_substr({itemid},'[^:]*$')) and I've tried a modified version of this, but to no luck.

Essentially our naming convention looks like the following: ONV266188 ABC Company Inc.

All I want is the ONVxxxxxx portion. there can be a different amount of numbers after the letters ONV, so trying a SUBSTR expression won't work as (from what I can see anyway) there is no way to make the length variable. I've tried regexp_substr({entityid},'[ONV(0-9)]^*')) thinking that I only wanted the ONV number and the digits that follow it, but this only returned the letter 'O'. Also, not sure if I'm using the syntax correctly as my assumption was using the caret and the astrisk would be stating "start from the beginning of the string". But even without those characters