Why does the third item 'c' appear in the return value of the regular expression /#\\/((\\w) )/.exec("/a-web/#/abc?")?

Understand the results of regular expression matching

This article will parse the return value of the regular expression /#\/((\\w) )/.exec("/a-web/#/abc?") in detail and explain the logic behind it.

Regular expressions /#\/((\\w) )/ are designed to match strings for a specific pattern. Let's analyze its return value step by step:

['#/abc', 'abc', 'c', index: 7, input: '/a-web/#/abc?', groups: undefined]

• ['#/abc'] : This is the entire matching string, and the regular expression successfully matches the fragment from #/ to the end abc .

• ['abc'] : This is the matching result of the first capture group (\w ) . The capture group matches one or more alphanumeric characters.

• ['c'] : This is the confusing part. This additional result is caused by (\w ) nesting in another capture group. (\w ) does a greedy match, matching as many characters as possible, and finally matching to abc . However, due to the implementation of the JavaScript engine, it will list the last matching character c in the capture group separately as the third result.

• index: 7 : Indicates the starting position of the matching string in the original string, starting from the 7th character.

• input: '/a-web/#/abc?' : Indicates the original input string.

• groups: undefined : The named capture group is not used, so the value is undefined .

The key to the problem is nested capture groups and greedy matching. To avoid such unnecessary results, it is recommended to modify the regular expression to avoid unnecessary nested capture groups. For example, you can use /#\/(\w )/ so that you can only capture abc without additional c .

Summarize:

Understanding the results of regular expression matching requires careful analysis of the structure of regular expressions, especially the greedy characteristics of the nesting of groups and the quantifiers. By adjusting regular expressions, you can obtain clearer and more expected matching results. It is recommended that when writing regular expressions, unnecessary nesting and complex structures should be avoided to improve readability and maintainability.

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