问题
I want to customize google re captcha to have only digits as option for verification code. Is is possible to do . If yes then how to do it ?
回答1:
No, this is not possible.
The reCaptcha system is meant as an easy way to plug in a Captcha to your site. As a side effect, it helps digitizing books by using image snippets from scans as a visual challenge. If only numbers should be displayed, then only such “words” from books could be used that are already known to be numbers. This would void the secondary objective of reCaptcha (helping the transcription of scans).
Using numbers only makes your system less secure (in theory). Whereas an attacking OCR software might have difficulty telling apart a g and 9, this now becomes unambiguous.
来源：https://stackoverflow.com/questions/18444933/customize-google-re-captcha-in-perl