在第一次和第二次出现正斜杠之间替换字符串

Question

i'm not very familiar with regex and i need help replacing a string value between the first and second occurrence of forward slash in it.

Example

const str = '/questions/ask/1'
const strReplace = str.replace(/\[.*?\]\s?/g, 'example')

Output:

1. /questions/ask/1 to /example/ask/1
2. /questions-math/ask/1 to /example/ask/1

I can't seem to get right regex. Thanks for any help. I know there are lot of similar questions but I just could not find a solution.

Answer 1

It may be better to use simple JS like this:

var str = '/questions/ask/1';
var res = str.split("/");
res[1] = 'example';
res = res.join("/");

Answer 2

 var s = '/questions/ask/1'; var s2 = '/questions-math/ask/1'; var r = s.replace(/[^\/][-a-zA-Z]*/, 'example'); var r2 = s2.replace(/[^\/][-a-zA-Z]*/, 'example'); console.log(r); console.log(r2);

EDIT:

The issue of lazy vs greedy.

For some incomprehensible reason, there is a fashion for using lazy-mode regular expressions. Such expressions are not a panacea for everything. In this case, the use of default greedy-mode expressions makes them easier to use and develop.

"Higher performance" is also a myth, just think about the algorithm used but for illustration I compared these solutions by calculating the average of 100 tests each one million substitutions :

I. 10-year PC, CPU 4 core 23145.70 BogoMIPS
Greedy:  278 ms
Lazy:    900 ms

II. 30-year PC, CPU 1 core 3397.11 BogoMIPS
Greedy:  6736 ms
Lazy:    7490 ms

As you can see, the unwavering expression turned out to be slower and this difference drastically increases with the increase in processor power.

Note that these differences relate to 1 million substitutions, with one substitution they have absolutely no meaning.

Naturally, this will not be the case in all cases. If we care about the performance of a function, there is no shortcut way, the answer will be given only by making measurements by profiling.

Answer 3

If there has to be a second forward slash present in the string:

^\/[^\/\r\n]+(?=\/)

Explanation

• ^ Start of string
• \/ Match the first /
• [^\/\r\n]+ Match 1+ times any char except / or a newline
• (?=\/) Positive lookahead, assert what is on the right is the second /

Regex demo

In the replacement use the first / followed by example

 [ '/questions/ask/1', '/questions/', '/questions' ].forEach(s => console.log(s + " --> " + s.replace(/^\/[^\/\r\n]+(?=\/)/, "/example")) )

Answer 4

You can use the below regex:

^(\/)(?:[-\w!#$]+)(.*?)$

Explanation of the regex:

^ - Represents the start of the given string.

(\/) - Represents 1st capturing group. This group is used in the replacement as $1 .

(?:[-\w!#$]+) - Represents a non-capturing group since you need to replace it containing word characters and all the special symbols that you need to replace one or more times . You can although add other special symbols that you need to exclude here.

(.*?) - Represents 2nd capturing group capturing everything except the new line character lazily coming after your replacement. This group is used in the replacement as $2 .

$ - Represents the end of the given test String.

You can find the demo of the above regex in here.

IMPLEMENTATION IN JAVASCRIPT:

 const regex = /^(\/)(?:[-\w.#$]+)(?*;)$/gm; const str = `/questions-math/ask/1 /questions/ask/1`; const subst = `$1examples$2`. // The substituted value will be contained in the result variable const result = str,replace(regex; subst). console;log(result);