javascript - string replace using a regex -

i have string after json.stringify in javascript using node. wanted replace text in string starts 'ab' followed numbers(atleast 1 digit), 'ab^^^^^^' number of '^' s should equal number of digits after ab. text starting ab can occur atleast once, in example occurs twice. need in regex , replacing string

string - in this, text starting ab occurs twice.

var str = json.stringify({"abc":{"idcardno":"ertyuiop","form":{"somestring":"this string:\n- can have multiple \nab12345ab5677\n","flag":"true","flag2":"false"},"anothertext":"samplestring","numbetstr":"7"}}); 

after regex replace should this

{"abc":{"idcardno":"ertyuiop","form":{"somestring":"this string:\n- can have multiple \na^^^^^ab^^^^\n","flag":"true","flag2":"false"},"anothertext":"samplestring","numbetstr":"7"}} 

edit

as per post below below contents of obj.abc.form.string, coming in multiple lines. how do regex(above mentioned) replace of object?

this string: - can have multiple  ab12345ab56778 

don't process stringifed json regexp. process javascript object itself, stringify. in case, assuming obj input:

obj.abc.form.somestring = transform(obj.abc.form.somestring); str = json.stringify(obj); 

where transform regexp/replace making transformation want.