Group fields of structure with same first n letters of field name

Hi, My data looks like this:

image1_control,
image1_drug,
image1_drug2,
image2_control,
image2_drug2,
image3_drug,
image4_control,
image4_drug2,

etc

These are all field names of a structure.

I would like to group field names based on the first 7 characters of the string. For example, I want to create a structure named image1 with subfields named control, drug1 and drug2.

I have tried using strcmp, but I would have to create a for loop to conduct a pairwise comparison. I'm having trouble using regexp and indexing all "images" with the matching names.

Can someone help me find a more efficient way to group field names that share the first 7 characters?

Thanks!

[Walter Roberson]

Duplicate merged into original https://www.mathworks.com/matlabcentral/answers/348110-matching-first-n-characters-of-string-in-array