Invalid regular expression error in javascript

Question

If the '[' special character is used in a regex constructor object, it throws Invalid regular expression: /mat[/: Unterminated character class

function findMatch() {
   var string="Find the match";
   var text = "Mat[";

   string.replace(new RegExp(text, 'gi'),  "found")
}

Why is this error occurring and how can I solve this?

Java or JavaScript? Looks like the latter... Anyways, the square bracket is a reserved character in a regex. You must escape it with a preceding back-slash. Also, avoid naming variables "string"; give is a more appropriate name like "originalText". — Mr. Polywhirl, Jun 22 '23 at 15:50
What that reserved character mean and how to escape it. Can you please explain me in detail. — Web Dev, Jun 22 '23 at 15:52
What is your desired output with this code? That expression (even if escaped), will replace nothing. — Mr. Polywhirl, Jun 22 '23 at 15:54
A backlash \ is usually the character to escape the one following, to make it a literal. [ has a meaning I regexp, which you don't want — sleepyhead, Jun 22 '23 at 15:56
https://stackoverflow.com/questions/3561493/is-there-a-regexp-escape-function-in-javascript — epascarello, Jun 22 '23 at 16:00
Why use regex at all, just use `string.replaceAll(text, "found")` — trincot, Jun 22 '23 at 17:55

phatfingers · Answer 1 · 2023-06-22T16:14:04.847

0

Regular expressions use the characters [] to identify a character class-- to match any character within the brackets. If you want to use either of those bracket characters as a literal, then you must escape it with a backslash. Your goal should be the regular expression:

/Mat\[/

You'll also need to escape the backslash itself to convert from the string to a regular expression. Thus, your code should be:

var text = "Mat\\[";

edited Jun 22 '23 at 16:14

answered Jun 22 '23 at 15:55

phatfingers

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incorrect, that will produce the same error. You are just escaping that character in the string, it will not be escaped in the reg exp. – epascarello Jun 22 '23 at 16:06
What is the correct to escape this – Web Dev Jun 22 '23 at 16:09
An oversight on my part. Corrected. – phatfingers Jun 22 '23 at 16:14
@WebDev — backslash is an escape in Javascript, so javascript sees "Mat\[" and makes the string "Mat[" ... the open-bracket was escaped. Now you pass that "Mat[" to `new RegExp(...)` and the regex _doesn't_ see a backslash, JS took it out when it created the String object. So regex sees `Mat[` and `[` is special, and it's missing a closing bracket `]`. To get the backslash to RegExp you have to escape the escape: `var text = "Mat\\["`. I see phatfingers corrected that while I was writing this, but thought a fuller explanation might be helpful. – Stephen P Jun 22 '23 at 16:17

score -2 · Answer 2 · edited Jun 22 '23 at 16:15

The error occurs because the square bracket character [ has a special meaning in regular expressions. It is used to define a character class, which allows specifying a set of characters that can match at that position.

In your example, the error is occurring because the regex pattern Mat[ is not properly terminated. The closing square bracket ] is missing, causing the error message "Unterminated character class."

To fix this issue, you need to escape the square bracket character using a backslash \ to treat it as a literal character instead of a special character in the regular expression.

This answer looks like ChatGPT – DavidW Jun 26 '23 at 08:03 — DavidW, Jun 26 '23 at 08:03

Invalid regular expression error in javascript

2 Answers2