Convert string to value in Custom Dimension as a Looker End User

[JoeSouthin]

As an end user of Looker, how can I convert a custom dimension string to a numeric value so I can aggregate the values?  I don't have access to LookML, advanced REGEX functions, cannot use CAST in custom dimension definition etc.  

I'm using combination of substring and position to extract number from a longer string, e.g. 

"this is a really long string with a value: 123 followed by some other text"

to return the value "123" as my custom dimension, and I'd like to sum this field, but am unable to whilst this is a string, not a numeric.

Closest I've got is to use the length of that string to get the closest order of magnitude as an approximation:

if(length($myvar})>0,power(10,length(${myvar})-1),0)