正则表达式无法按预期方式工作/(d)。\\1/

Question

I am beginner in RegEx so I am reading the info page of regEx on stackoverflow.

eg: /(d).\\1/ matches and captures 'dad' in "abcdadef" while /(?:.d){2}/ matches but doesn't capture 'cdad'.

I tried :-

var pattern=/(d).\1/


var val="abcdadef";

console.log(pattern.exec(val));

It shows array of ["dad","d"] but i don't know why ? As said in info it just only capture the "dad" why it is capturing two values in array?. And what is the use of '\\1' in the end of pattern ?

Please provide me more info how to use it.

Thanks :-)

Answer 1

when you use (), you're telling regex to match the in between () and store it as a capturing group. Each match will have capturing groups of its own. Try your expression here . A regex match object is normally a collection that contains the entire match of the regex followed by capturing groups of that match.

Edit: As per your comment below, here's an another pattern (m).\\1 and the text upon which we're executing the regex is mum. In this example, regex will attempt to do the following:

1. match the literal m and hence we used () , it's going to store the match in a capturing group. This capturing group will make it to the match collection later.
2. . will match any character other than newline so in our case, it will match the literal u .
3. \\1 will attempt to match the next character using the first matching group as a pattern and that would be the literal m in our case.

The final result will be the regex match of mum and the only capturing group would be m .