'? ' - Regular expression - js

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let american = "color";
let rainbowRegex= /colou?r/;
rainbowRegex.test(american); // Returns true -- why?

What I have tried:

to understand and try another examples. but still, i understand nothing

Posted 10-Mar-19 23:42pm

Member 14105304

Updated 10-Mar-19 23:53pm

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OriginalGriff · Answer 1 · 2019-03-10T23:47:00

Solution 1

Because '?' on it's own (without '*' or '+' in front of it) says "zero or one" of the preceding element.
So "colou?r" matches either "color" or "colour" but not "colotr".

As I suggested last time, get a copy of Expresso[^] - it's free, and it examines and generates Regular expressions.
It would have explained this to you!

Posted 10-Mar-19 23:47pm

OriginalGriff

v2

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Member 14105304 11-Mar-19 5:52am

so the '?' just dismiss the behind word of it and then checks?

OriginalGriff 11-Mar-19 6:00am

No, read what I said:

'?' on it's own ... says "zero or one" of the preceding element.

Since the preceding element is a single character 'u' it says "zero or one 'u'"
To "disregard a word" you'd have to make it a group:
"(colou)?r" would match "colour" or "r" because "colou" is a group and that acts as the preceding element for the '?'

Seriously, download Expresso and try it - it will help you understand!

Wastedtalent · Answer 2 · 2019-03-10T23:47:00

Solution 2

The u is optional in the regex, as denoted by the ?

So both 'color' and 'colour' will match.

Type colou?r in to the regex box on here to see:

Debuggex: Online visual regex tester. JavaScript, Python, and PCRE.[^]

Posted 10-Mar-19 23:47pm

Wastedtalent

Updated 10-Mar-19 23:48pm

v2

Patrice T · Answer 3 · 2019-03-10T23:53:00

Quote:
'? ' - Regular expression - js

Advice: read documentation.

Here is a link to RegEx documentation:
perlre - perldoc.perl.org[^]
Here is links to tools to help build RegEx and debug them:
.NET Regex Tester - Regex Storm[^]
Expresso Regular Expression Tool[^]
RegExr: Learn, Build, & Test RegEx[^]
Online regex tester and debugger: PHP, PCRE, Python, Golang and JavaScript[^]
This one show you the RegEx as a nice graph which is really helpful to understand what is doing a RegEx: Debuggex: Online visual regex tester. JavaScript, Python, and PCRE.[^]
This site also show the Regex in a nice graph but can't test what match the RegEx: Regexper[^]