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Say I have a string "ldhjshjds HdAjhdshj4 Hdsshj4 kdskjdshjdsjds"

I only want to search for substrings (alphanumeric only) starting with "H", but only if the string is between 10-20 characters.

"HdAjhdshj4" would be a match. "Hdsshj4" would not.

Would such a regex be costly on CPU cycles?

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r"\bH[A-Za-z0-9]{9,19}\b" looks for precisely that.

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3 Comments

It should probably be r"\bH[A-Za-z0-9]{9,19}\b"; without the word anchors, it won't insist on the "word" starting with an H, and it won't really enforce the length limit (it will match the first 20 characters of a 30 character word, which does not seem the be desired). Also, side-note, you've got a typo in the character class; you typed a-Z instead of a-z.
Touché, from the examples given it does not seem like OP desires matches inside of words. Edited the answer to your expression.
Oh, and just to answer the OP's other question: With the word and character anchors and bounded quantifiers, this regex should be pretty darn cheap to run. Profile if you have performance issues, but I don't foresee any.
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Use negative lookarounds.

re.findall(r'(?<!\S)H[A-Za-z0-9]{9,19}(?!\S)', s)

DEMO

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You can do this using lookarounds.

re.findall(r'(?:^|(?<=\s))H[A-Za-z0-9]{9,19}(?=\s|$)', s)
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