Regular expression number extraction from string
===

Have a quick question about a regular expression.

I'm looking to see if the following string(s) match the number 1 in them

23_13_15_145_34_1_135

23_13_15_145_34_1

1_23_13_15_145_3

I only want to match it exactly so the first example must not include the 13, 15 or 145, just the 1.

Same with the start and end examples with the 1 at the start and end of the string.

Thanks in advance.