Letter case and look-alike characters
2026-01-21 · Captchas
O and 0, l and 1, S and 5. The options decide which way those characters fold.
Captcha fonts pick characters that look alike on purpose. A reader that always turns O into 0 will ruin a word captcha, and a reader that keeps O will ruin a digit code.
numeric: true with no letter option folds look-alikes into digits. uppercase: true alone folds the other case into capitals. lowercase: true alone folds into small letters. Set both cases, or neither, when you need the case kept as it was drawn.
length stacks with those rules. A six-digit code is numeric: true and length: 6. The decoder will not return five digits or seven.
If you are unsure what the portal allows, look at one sample. A code that is only digits should be marked numeric. A code that mixes letters should not.
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