为什么此正则表达式[^ \\ /] \\ d {4}与行首的4位数字不匹配？

Question

The pattern [^\\/]\\d{4} works for 2014(i) and 01/01/2015 but not 2014(i) when it is the first item in the text file.

Sample in regexstorm

Answer 1

You are looking for a character before digits and at the beginning of line / string there is no character right before them. As @JonSkeet mentioned in comments since no character before digits is found at beginning of a line, [^\\/] consumes first digit so number of left digits doesn't match \\d{4} . That said, you are looking for this:

(?<!\/)\d{4}

• (?<!\\/) a negative lookbehind which assures no / before digits (an assertion, no character consuming applies)

Live demo

or if you have no problem with matching preceding character:

(?:^|[^\/])\d{4}