Validating forms (and more) with the HTML5 pattern attribute

Validating Forms
(AND MORE)
WITH THE

HTML5 Pattern
Attribute

@cliener

HTML5 RECAP

http://wufoo.com/html5/
Before I get stuck into patterns, I’ll quickly go through some of the important parts of HTML5 forms. If you want to play along at
home, have a look at Wufoo’s HTML Forms guide.

HTML5 RECAP
<input
type="email">

Most of the new input types come with awesome custom keyboards for iOS and more recent Android (amongst others). Some
browser versions offer basic validation in but inconsistent implementations mean they can’t be relied upon.
The key new inputs types are “email”,

HTML5 RECAP
<input
type="tel">

“tel”,

HTML5 RECAP
<input
type="url">

“url” and

HTML5 RECAP
<input
type="number">

“number”.

HTML5 RECAP
<input
type="datetime">

Unfortunately “datetime”, “time” and “year” input types have been marked as “in danger” by the W3C due to poor and
inconsistent browser support. There’s something of a chicken and egg situation when it comes to standards and browser
support. Unfortunately for the HTML5 date and time inputs,

the chicken looks something like this.

HTML5 RECAP
<input
required>

Important attributes to note are “required”,

HTML5 RECAP
<input
placeholder=

"dd/mm/yyyy">

“placeholder”

PATTERNS
<input
pattern="/d*">

and, of course, “pattern”. Patterns allow you to provide a regular expression string primarily for input validation. A numeric
regulation expression like the this one can trigger a numeric keyboard. You’ll often see the above pattern as “[0-9]*” because
most people don’t know how to write regular expressions to save themselves.

NOW YOU HAVE TWO PROBLEMS

http://evolt.org/node/36435
I can’t remember the origin so I’ll call it an old saying: “You have a problem which can be solved with a regular expression. Now
you have two problems.”
I won’t pretend regular expressions are the easiest to write however a good guide the one on Evolt will certainly make life easier
for you.

PATTERNS
/d*/
.test("12345");
>>>
true

I ﬁnd the JS console (in a browser near you) are a handy way to test out regular expressions. This one matches any numeric value

PATTERNS
/d*/
.test("Once
I
caught
a

fish
alive");
>>>
false

And correctly returns false to a string.

POSTCODE
/d{4}/
.test("605");
>>>
false

Australian postcodes are easily matched with this pattern - exactly four digits - returning false here for “605”

POSTCODE
/d{4}/
.test("6050");
>>>
true

and true here for a correct value.

YEAR
/d{4}|d{2}/
.test("13");
>>>
true

Getting slightly more complex with a pipe which acts as an “or” switch, we can match either two or four digit years.

TIME
/(([0-‐1]d)|(2[0-‐3]))
:[0-‐5]d/
.test("08:37");
>>>
true

Square brackets specify a range of values, round brackets group sub-expressions together: here matching a twenty hour time
value.

TIME
/(([0-‐1]?d)|(2[0-‐3]))
(:|.)[0-‐5]d/
.test("8.37");
>>>
true

With a slight modiﬁcation, we’re now matching a wider range of natural inputs.

DATE
/d{2}/d{2}/d{4}/
.test("24/10/2013");
>>>
true

Dates seem fairly straight-forward

DATE
/d{2}/d{2}/d{4}/
.test("99/99/9999");
>>>
true

but are a lot more complex than they seem.

DATE
/(?=d)(?:(?:31(?!.(?:0?[2469]|11))|(?:
30|29)(?!.0?2)|29(?=.0?2.(?:(?:(?:1[6-‐9]|
[2-‐9]d)?(?:0[48]|[2468][048]|[13579]
[26])|(?:(?:16|[2468][048]|[3579]
[26])00)))(?:x20|$))|(?:2[0-‐8]|1d|0?
[1-‐9]))([-‐/])(?:1[012]|0?[1-‐9])1(?:
1[6-‐9]|[2-‐9]d)?dd(?:(?=x20d)x20|
$))?/
.test("99/99/9999");
>>>
false
A bit more work and we end up with this.

At this point I can understand if you’re a little shocked, so I’ll pull it back a little.

EMAIL
/w+@w+.w+/
.test("a@b.c")
>>>
true

Email addresses are often something of a mine ﬁeld but, on the surface, the pattern is fairly simple: “a@b.c”

EMAIL
/w+@w+.w+/
.test("cli+ner@

server.com.au");
>>>
true

Already we’re matching better than most email validation rules out there.

EMAIL
/w+@w+.w+/
.test("1337@server.com");
>>>
true

Except the simplicity soon gets us unstuck when we get false positives. Given how scary date validation was, it’s tempting to see
what Google has to offer

EMAIL
/(?:(?:rn)?[
t])*(?:(?:(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[
t]))*"(?:(?:rn)?[
t])*)
(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[
t]))*"(?:(?:rn)?
[
t])*))*@(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*)(?:.(?:(?:r
n)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*))*|(?:[^()<>@,;:".[]
000-‐
031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[
t]))*"(?:(?:rn)?[
t])*)*<(?:(?:rn)?[
t])*(?:@(?:[^()<>@,;:".[]
000-‐
031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:r
n)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*))*(?:,@(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?
=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".
[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*))*)*:(?:(?:rn)?[
t])*)?(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r
]|.|(?:(?:rn)?[
t]))*"(?:(?:rn)?[
t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r
]|.|(?:(?:rn)?[
t]))*"(?:(?:rn)?[
t])*))*@(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|
.)*](?:(?:rn)?[
t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?
[
t])*))*>(?:(?:rn)?[
t])*)|(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[
t]))*"(?:(?:rn)?
[
t])*)*:(?:(?:rn)?[
t])*(?:(?:(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[
t]))*"(?:(?:r
n)?[
t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[
t]))*"(?:(?:r
n)?[
t])*))*@(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*)(?:.(?:
(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*))*|(?:[^()<>@,;:".[]

000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[
t]))*"(?:(?:rn)?[
t])*)*<(?:(?:rn)?[
t])*(?:@(?:[^()<>@,;:".[]

000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:
(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*))*(?:,@(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|
Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:
".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*))*)*:(?:(?:rn)?[
t])*)?(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^
"r]|.|(?:(?:rn)?[
t]))*"(?:(?:rn)?[
t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^
"r]|.|(?:(?:rn)?[
t]))*"(?:(?:rn)?[
t])*))*@(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]
r]|.)*](?:(?:rn)?[
t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:
rn)?[
t])*))*>(?:(?:rn)?[
t])*)(?:,s*(?:(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?
[
t]))*"(?:(?:rn)?[
t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?
[
t]))*"(?:(?:rn)?[
t])*))*@(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?
[
t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*))*|(?:
[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[
t]))*"(?:(?:rn)?[
t])*)*<(?:(?:rn)?[
t])*(?:@(?:
[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]

000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*))*(?:,@(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:
(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|
Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*))*)*:(?:(?:rn)?[
t])*)?(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[
["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[
t]))*"(?:(?:rn)?[
t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[
["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[
t]))*"(?:(?:rn)?[
t])*))*@(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[
["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*)(?:.(?:(?:rn)?[
t])*(?:[^()<>@,;:".[]
000-‐031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".
[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*))*>(?:(?:rn)?[
t])*))*)?;s*)/
.test("ha@ha.ha");
>>>
true

http://www.codinghorror.com/blog/2005/02/regex-use-vs-regex-abuse.html
And here is 6.2KB of RFC standard-compliant email regular expression.

Which brings me to a major problem when regular expressions.

Unfortunately, a fair number of programmers out in the wild haven’t quite worked out regular expressions yet so you need to be
really careful about what you’re write.

EMAIL
/([w!#%&$'*+-‐=?^_`{|}~]+[w!#
%&$'*+-‐=?^_`{|}~.]+[w!#%&$'*+
-‐=?^_`{|}~]+|[w!#%&$'*+-‐=?
^_`{|}~]{1,2})@([A-‐Za-‐z0-‐9-‐]+.)
+[A-‐Za-‐z0-‐9-‐]+/
.test("1337@server.com");
>>>
false
For the record, here’s my RFC-compliant email regular expression.

SECURITY

As a whole, password validation is awful.

SECURITY

Over-validation is a great way to make people hate you

SECURITY

because, as much as you might think otherwise, you don’t know as much about them as you think you do.

SECURITY

But then there are people like, presumably, Bob.

And here be dragyns. In the last couple of years, millions of passwords - hashed and otherwise - have been grabbed from
various online services which means that everything we though we knew about password validation is, in essence, wrong. This
also means that enforced password validation is probably wrong too. So, please don’t. Invest your energy into two factor
authentication or something else that’s actually secure rather than incurring the wrath of the users who do know what they’re
doing.

CREDIT CARDS
"4123
5678
9012
3456"
.replace(/(s)/g,
"");
>>>
"4123567890123456"

While I’ve shown how awesome regular expressions are for pattern validation, they can also manipulate data. Right here is
everything you need to strip spaces from a credit card number so I can enter it with spaces as it is on my card. You no longer
have any excuses.

ERROR MESSAGES
<input
pattern="d{4}"

title="Please
enter
a

valid
postcode
e.g.

3000">

Rather crucial when doing pattern validation is presenting a meaningful error message to users. In HTML5, browsers are abusing
the title attribute to this end. You can also set the .setCustomValidity() DOM property but title will work for most instances.

ERROR MESSAGES

The results aren’t perfect, but they’re a whole lot better than server validation.

In summary, are regular expressions better than kittens? I think the answer is deﬁnitively yes.

@cliener
http://www.slideshare.net/cliener/

FURTHER READING
HTML5 Rocks: Constraint Validation: Native Client
Side Validation for Web Forms
http://www.html5rocks.com/en/tutorials/
forms/constraintvalidation/

Validating forms (and more) with the HTML5 pattern attribute

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