Regex Pattern between two characters which has multiple /n

[rajat_dhammi (rajat)]

Hello, I want to extract
1a
b
From the below string. I know data manipulation methods, can someone suggest regex pattern for that.
Please note that a and b are just examples , there could be paragraph or line of alphanumeric words instead of a and b, so we should not consider a and b in the string
I want everything starting from 1 (Including 1) before 2

“This is Test Data
1 a
b
2 a
b
3 a
b”

[ppr (Peter | MVP)]

or Anchored to the 2

so we would dynamize a and b by variables to be flexible for the other searches
String.Format("{0}(.|\n)*?{1}", var1, var2)

[rajat_dhammi (rajat)]

Thank @ppr
With this regex I am able to extract “1(.|\n)*?(?=2 )”
1 a
b

What if I want to extract
2 a
b

and
3 a
b

[ppr (Peter | MVP)]

@rajat_dhammi

What if I want to extract
2 a
b

and
3 a
b

could be linked to:

@rajat_dhammi

Please note that a and b are just examples , there could be paragraph or line of alphanumeric words instead of a and b

maybe the requiremetns are to resharpen