Modifying a RegEx pattern to work whether or not there is a space between two elements.

s*\d{1,3}\s*[A-M]

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will give you a match if there is ANY whitespace between the numbers and letters

s*\d{1,3}\s{0,1}[A-M]

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will give you a match if there are exactly zero or one whitespace characters between the numbers and letters

If you're specifically looking for the space character (ascii value 32) and not any whitespace (which can include tabs, new lines, or other whitespace depending on regex options) then you could do this:

s*\d{1,3}[ ]{0,1}[A-M]

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Or even more explicitly, call out the octal ASCII value of the space character like this:

s*\d{1,3}\40{0,1}[A-M]

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I could probably keep coming up with variants that would serve your needs. Regex is extremely flexible. Let me know if you need more than above.

By the way, the Regex you provided would only match "18A" on the string "18ABC". It would not match "18ABC". For that you'd need to include a trailing asterisk which means [zero or more repetitions].