PHP Pattern Validation

Question

I'm having a bit of trouble getting my pattern to validate the string entry correctly. The PHP portion of this assignment is working correctly, so I won't include that here as to make this easier to read. Can someone tell me why this pattern isn't matching what I'm trying to do?

This pattern has these validation requirements:

Should first have 3-6 lowercase letters
This is immediately followed by either a hyphen or a space

Followed by 1-3 digits

$codecheck = '/^([[:lower:]]{3,6}-)|([[:lower:]]{3,6} ?)\d{1,3}$/';

Currently this catches most of the requirements, but it only seems to validate the minimum character requirements - and doesn't return false when more than 6 or 3 characters (respectively) are entered.

Thanks in advance for any assistance!

Instead of the "or" pipe, use [\s-] to look for a - or a space. — kainaw
– kainaw, Commented Mar 22, 2016 at 19:45
You don't need to duplicate the lower /^[[:lower:]]{3,6}[\s-]\d{1,3}$/ — AbraCadaver
– AbraCadaver, Commented Mar 22, 2016 at 19:55
I don't think design-patterns tag is appropriate here. You should maybe use regex instead. — TerraPass
– TerraPass, Commented Mar 22, 2016 at 19:55
@AbraCadaver, please write it as an answer and not a comment. — ndnenkov
– ndnenkov, Commented Mar 22, 2016 at 20:11

Wiktor Stribiżew · Accepted Answer · 2016-03-22 20:14:02Z

4

The problem here lies in how you group the alternatives. Right now, the regex matches a string that

^([[:lower:]]{3,6}-) - starts with 3-6 lowercase letters followed with a hyphen
| - or
([[:lower:]]{3,6} ?)\d{1,3}$ - ends with 3-6 lowercase letters followed with an optional space and followed with 1-3 digits.

In fact, you can get rid of the alternation altogether:

$codecheck = '/^\p{Ll}{3,6}[- ]\d{1,3}$/';

See the regex demo

Explanation:

^ - start of string
\p{Ll}{3,6} - 3-6 lowercase letters
[- ] - a positive character class matching one character, either a hyphen or a space
\d{1,3} - 1-3 digits
$ - end of string

answered Mar 22, 2016 at 20:14

Wiktor Stribiżew

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3 Comments

Wiktor Stribiżew Over a year ago

@Shaw: Just to clarify that your regex can work, too, once the alternations are placed into a group: ^(?:([[:lower:]]{3,6}-)|([[:lower:]]{3,6} ?))\d{1,3}$. However, it is not efficient.

Shaw Over a year ago

Thanks for pointing that out - as I'm just learning it, it's good to know I was on the right track for educational purposes, but not yet seeing the whole picture in regards to efficiency.

Wiktor Stribiżew Over a year ago

When using alternation like this, backtracking is increasing the number of steps needed to return a match/or detect a non-matching string. See the green box showing number of steps with your regex and mine. Although that number of steps is not a direct indicator of performance, when the step count is doubled like in this case, it hints at performance difference between the two patterns.

trincot · Accepted Answer · 2016-03-22 20:15:24Z

2

You need to delimit the scope of the | operator in the middle of your regex.

As it is now:

the right-side argument of that OR runs up until the very end of your regex, even including the $. So the digits, nor the end-of-string condition do not apply for the left side of the |.
the left-side argument of the OR starts with ^, and only applies to the left side.

That is why you get a match when you supply 7 lowercase characters. The first character is ignored, and the rest matches with the right-side of the regex pattern.

answered Mar 22, 2016 at 20:15

trincot

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1 Comment

Shaw Over a year ago

Thanks for breaking it down!

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