regex expression to find strings with two letters

I need to find all strings where strings have letters "H" and "M" but no other letters before or after but other symbols are ok.
Valid strings:

HM
(HM)
&HM%
This is HM
HM are two letters

Invalid strings:

Marshmellows
asdfHMASDF
sfafhmasdf

>Solution :

You may use this regex in ignore case mode:

/^(?:.*?[^a-z\n])?HM(?:[^a-z\n].*)?$/igm

RegEx Details:

• ^: Start
• (?:.*?[^a-z\n])?: Match an optional match of anything followed by a non-letter
• HM: Match HM (ignore case)
• (?:[^a-z\n].*)?: Match an optional non-letter followed by anything till end
• $: End

or else using look arounds:

/^.*?(?<![a-z])HM(?![a-z]).*/igm

RegEx Demo