regular - javascript regex test




Converting user input string to regular expression (6)

I am designing a regular expression tester in HTML and JavaScript. The user will enter a regex, a string, and choose the function they want to test with (e.g. search, match, replace, etc.) via radio button and the program will display the results when that function is run with the specified arguments. Naturally there will be extra text boxes for the extra arguments to replace and such.

My problem is getting the string from the user and turning it into a regular expression. If I say that they don't need to have //'s around the regex they enter, then they can't set flags, like g and i. So they have to have the //'s around the expression, but how can I convert that string to a regex? It can't be a literal since its a string, and I can't pass it to the RegExp constructor since its not a string without the //'s. Is there any other way to make a user input string into a regex? Will I have to parse the string and flags of the regex with the //'s then construct it another way? Should I have them enter a string, and then enter the flags separately?


Here is a one-liner: str.replace(/[|\\{}()[\]^$+*?.]/g, '\\$&')

I got it from the escape-string-regexp NPM module.

Trying it out:

escapeStringRegExp.matchOperatorsRe = /[|\\{}()[\]^$+*?.]/g;
function escapeStringRegExp(str) {
    return str.replace(escapeStringRegExp.matchOperatorsRe, '\\$&');
}

console.log(new RegExp(escapeStringRegExp('example.com')));
// => /example\.com/

I suggest you also add separate checkboxes or a textfield for the special flags. That way it is clear that the user does not need to add any //'s. In the case of a replace, provide two textfields. This will make your life a lot easier.

Why? Because otherwise some users will add //'s while other will not. And some will make a syntax error. Then, after you stripped the //'s, you may end up with a syntactically valid regex that is nothing like what the user intended, leading to strange behaviour (from the user's perspective).


Thanks to earlier answers, this blocks serves well as a general purpose solution for applying a configurable string into a RegEx .. for filtering text:

var permittedChars = '^a-z0-9 _,[email protected]+<>';
permittedChars = '[' + permittedChars + ']';

var flags = 'gi';
var strFilterRegEx = new RegExp(permittedChars, flags);

log.debug ('strFilterRegEx: ' + strFilterRegEx);

strVal = strVal.replace(strFilterRegEx, '');
// this replaces hard code solt:
// strVal = strVal.replace(/[^a-z0-9 _,[email protected]+]/ig, '');

This will work also when the string is invalid or does not contain flags etc:

function regExpFromString(q) {
  let flags = q.replace(/.*\/([gimuy]*)$/, '$1');
  if (flags === q) flags = '';
  let pattern = (flags ? q.replace(new RegExp('^/(.*?)/' + flags + '$'), '$1') : q);
  try { return new RegExp(pattern, flags); } catch (e) { return null; }
}

console.log(regExpFromString('\\bword\\b'));
console.log(regExpFromString('\/\\bword\\b\/gi'));
            

Use the RegExp object constructor to create a regular expression from a string:

var re = new RegExp("a|b", "i");
// same as
var re = /a|b/i;

You can ask for flags using checkboxes then do something like this:

var userInput = formInput;
var flags = '';
if(formGlobalCheckboxChecked) flags += 'g';
if(formCaseICheckboxChecked) flags += 'i';
var reg = new RegExp(userInput, flags);




regex