Regular Expression for alphanumeric and underscores

To match a string that contains only those characters (or an empty string), try

"^[a-zA-Z0-9_]*$"

This works for .NET regular expressions, and probably a lot of other languages as well.

Breaking it down:

^ : start of string
[ : beginning of character group
a-z : any lowercase letter
A-Z : any uppercase letter
0-9 : any digit
_ : underscore
] : end of character group
* : zero or more of the given characters
$ : end of string

If you don't want to allow empty strings, use + instead of *.

As others have pointed out, some regex languages have a shorthand form for [a-zA-Z0-9_]. In the .NET regex language, you can turn on ECMAScript behavior and use \w as a shorthand (yielding ^\w*$ or ^\w+$). Note that in other languages, and by default in .NET, \w is somewhat broader, and will match other sorts of Unicode characters as well (thanks to Jan for pointing this out). So if you're really intending to match only those characters, using the explicit (longer) form is probably best.

There's a lot of verbosity in here, and I'm deeply against it, so, my conclusive answer would be:

/^\w+$/

\w is equivalent to [A-Za-z0-9_], which is pretty much what you want. (unless we introduce unicode to the mix)

Using the + quantifier you'll match one or more characters. If you want to accept an empty string too, use * instead.

You want to check that each character matches your requirements, which is why we use:

[A-Za-z0-9_]

And you can even use the shorthand version:

\w

Which is equivalent (in some regex flavors, so make sure you check before you use it). Then to indicate that the entire string must match, you use:

^

To indicate the string must start with that character, then use

$

To indicate the string must end with that character. Then use

\w+ or \w*

To indicate "1 or more", or "0 or more". Putting it all together, we have:

^\w*$

Um...question: Does it need to have at least one character or no? Can it be an empty string?

^[A-Za-z0-9_]+$

Will do at least one upper or lower case alphanumeric or underscore. If it can be zero length, then just substitute the + for *

^[A-Za-z0-9_]*$

Edit:

If diacritics need to be included (such as cedilla - ç) then you would need to use the word character which does the same as the above, but includes the diacritic characters:

^\w+$

Or

^\w*$

How about:

^([A-Za-z]|[0-9]|_)+$

...if you want to be explicit, or:

^\w+$

...if you prefer concise (Perl syntax).

This works for me, found this in the O'Reilly's "Mastering Regular Expressions":

/^\w+$/

Explanation:

• ^ asserts position at start of the string
  • \w+ matches any word character (equal to [a-zA-Z0-9_])
  • "+" Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)
• $ asserts position at the end of the string

Verify yourself:

const regex = /^\w+$/;
const str = `nut_cracker_12`;
let m;

if ((m = regex.exec(str)) !== null) {
    // The result can be accessed through the `m`-variable.
    m.forEach((match, groupIndex) => {
        console.log(`Found match, group ${groupIndex}: ${match}`);
    });
}

The following regex matches alphanumeric characters and underscore:

^[a-zA-Z0-9_]+$

For example, in Perl:

#!/usr/bin/perl -w

my $arg1 = $ARGV[0];

# check that the string contains *only* one or more alphanumeric chars or underscores
if ($arg1 !~ /^[a-zA-Z0-9_]+$/) {
  print "Failed.\n";
} else {
    print "Success.\n";
}

This should work in most of the cases.

/^[\d]*[a-z_][a-z\d_]*$/gi

And by most I mean,

abcd       True
abcd12     True
ab12cd     True
12abcd     True

1234       False

Explanation

1. ^ ... $ - match the pattern starting and ending with
2. [\d]* - match zero or more digits
3. [a-z_] - match an alphabet or underscore
4. [a-z\d_]* - match an alphabet or digit or underscore
5. /gi - match globally across the string and case-insensitive

For those of you looking for unicode alphanumeric matching, you might want to do something like:

^[\p{L} \p{Nd}_]+$

Further reading at http://unicode.org/reports/tr18/ and at http://www.regular-expressions.info/unicode.html

Here is the regex for what you want with a quantifier to specify at least 1 character and no more than 255 characters

[^a-zA-Z0-9 _]{1,255}

I believe you are not taking Latin and Unicode characters in your matches. For example, if you need to take "ã" or "ü" chars, the use of "\w" won't work.

You can, alternatively, use this approach:

^[A-ZÀ-Ýa-zà-ý0-9_]+$

Hope it helps!

^\w*$ will work for below combinations

1
123
1av
pRo
av1

For Java, Only case insensitive alphanumeric and underscore are allowed.

• ^ Matches the string starting with any characters
• [a-zA-Z0-9_]+ Matches alpha-numeric character and underscore.
• $ Matches the string ending with zero or more characters.

public class RegExTest {
    public static void main(String[] args) {
        System.out.println("_C#".matches("^[a-zA-Z0-9_]+$"));
    }
}

To check the entire string and not allow empty strings, try

^[A-Za-z0-9_]+$

Required Format Allow these 3:

1. 0142171547295
2. 014-2171547295
3. 123abc

Don't Allow other formats:

validatePnrAndTicketNumber(){
    let alphaNumericRegex=/^[a-zA-Z0-9]*$/;
    let numericRegex=/^[0-9]*$/;
    let numericdashRegex=/^(([1-9]{3})\-?([0-9]{10}))$/;
   this.currBookingRefValue = this.requestForm.controls["bookingReference"].value;
   if(this.currBookingRefValue.length == 14 && this.currBookingRefValue.match(numericdashRegex)){
     this.requestForm.controls["bookingReference"].setErrors({'pattern': false});
   }else if(this.currBookingRefValue.length ==6 && this.currBookingRefValue.match(alphaNumericRegex)){
    this.requestForm.controls["bookingReference"].setErrors({'pattern': false});
   }else if(this.currBookingRefValue.length ==13 && this.currBookingRefValue.match(numericRegex) ){
    this.requestForm.controls["bookingReference"].setErrors({'pattern': false});
   }else{
    this.requestForm.controls["bookingReference"].setErrors({'pattern': true});
   }
}

<input name="booking_reference" type="text" [class.input-not-empty]="bookingRef.value"
    class="glyph-input form-control floating-label-input" id="bookings_bookingReference"
    value="" maxlength="14" aria-required="true" role="textbox" #bookingRef
    formControlName="bookingReference" (focus)="resetMessageField()" (blur)="validatePnrAndTicketNumber()"/>