正则表达式匹配特定位置的数字

Question

I want to match a string using regex, the string I wanna match is xxxxx_S_xxxxxx (where x are numbers), I want to match the three numbers closer to S to the left and the four numbers closer to the right.

Let's say I have c1123_S_1234 so I want to match 123_S_1234

This is what I tried:

(^[0-9]{3}$+_S_[0-9])

but it's not working, not matching anything, any suggestions?

Answer 1

Regex engine parses the text from left to right, ie it checks the position at the start of string for a match first, if it does not match, it goes on to the right and checks the rest of the string. The pattern sequences are also parsed from left to right, if you have (^[0-9]{3}$+_S_[0-9]) , first, the ^ will be checked, ie the start of string. And as there is only one start of string, the regex could potentially only match at the start of string, but the $ end of string anchor presence in the middle of the regex makes it a pattern that matches no string.

That means you can't tell the regex engine to find a _S_ substring and then start consuming text to the right or left.

You need

[0-9]{3}_S_[0-9]{4}
\d{3}_S_\d{4}

See the regex demo . [0-9]{3} searches for three consecutive digits, then there must be _S_ right after, and then any four digits must follow.