Example:

Input: **Type_1_deiodinase** , **D1** , metabolizes different forms, **A** , **B** of thyroid hormones to control levels of T3 , the active ligand for **thyroid_hormone_receptors** , **TR**

Output: **Type_1_deiodinase_(D1)** metabolizes different forms, **A_(B)** of thyroid hormones to control levels of T3 , the active ligand for **thyroid_hormone_receptors_(TR)**

If I do the following:  $line =~ s/\*\*([^\*]+)\*\*\s\,\s\*\*([^\*]+)\*\*(\s\,)?/**$1_($2)**/g;

it merges all $1 and $2 in the sentence. However, I want to merge terms only if either of $1 or $2 contains '_'.

That is in the above sentence, **Type_1_deiodinase_(D1)** and **thyroid_hormone_receptors_(TR)** are OK, while **A_(B)** is not. Is there a way to apply 'if' condition in the substitution expression above so that I can merge only those adjacent terms that contain '_'?

Thanks a lot.