.NET 正则表达式从混合字符值中提取 1-4 位数字值

Question

What I am trying to accomplish a .NET regular expression for the following strings:

Example 1. abc1234abc - 1234
Example 2. abc12345abc - no results
Example 3. abc1234abc12345 - 1234

I am trying to extract a 1-4 numeric ( 1 , 12 , 123 , or 1234 ) from any mixed character value but no more than 4 digits (IE. NOT 12345 ).

The [0-9]{1,4} pattern yields:

Example 1. abc1234abc - 1234 (good)
Example 2. abc12345abc - 1234, (should return no results)
Example 3. abc1234abc12345 - 1234, 1234, 5 (should return ONLY 1234)

What am I missing? Thank you for the help, I am very much new to regular expressions and I could not find what I am looking for anywhere. I hope my question makes sense.

Answer 1

You are missing the lookarounds that will check the number is not enclosed with more digits:

(?<!\d)\d{1,4}(?!\d)

Or (if you need to only match regular digits excluding all Uncide ones in Hindi, etc.):

(?<![0-9])[0-9]{1,4}(?![0-9])

See regex demo

The (?<!\\d) is a negative lookbehind that checks if the next subpattern is not preceded with some other subpattern (looks before the current position, "looks behind"), and (?!\\d) is a negative lookahead that makes sure that there is no digit after the current position ("looks ahead"). Lookarounds do not consume characters, they just check and then return true or false (thus allowing or failing a match), so, you only get your expected match as a result.

More details on regex lookarounds at regular-expressions.info