Question

I have such input string:

left/1234567890

regular expression:

(left(?<=/)[0-9]{10}?)

I want to get such result: 1234567890.
But it doesn’t work.

Although, the next input string:

/1234567890

with the next regular expression:

((?<=/)[0-9]{10}?)

get result as expected: 1234567890.

Answer 1

This is because you did not include left into your lookbehind:

((?<=left/)[0-9]{10}?)

In your first example, you match left then the regex engine’s ‘pointer’ is between t and /, so your lookbehind cannot match, because the regex engine has not passed the slash yet.