RegEx to find a PHP RegEx string - php

Question

I want to match a PHP regex string.
From what I know, they are always in the format (correct me if I am wrong):
/ One opening forward slash
the expression Any regular expression
/ One closing forward slash
[imsxe] Any number of the modifiers NOT REPEATING
My expression for this was:
^/.+/[imsxe]{0,5}$
Written as a PHP string, (with the open/close forward slash and escaped inner forward slashes) it is this:
$regex = '/^\/.+\/[imsxe]{0,5}$/';
which is:
^ From the beginning
/ Literal forward slash
.+ Any character, one or more
/ Literal forward slash
[imsxe]{0,5} Any of the chars i,m,s,x,e, 0-5 times (only 5 to choose from)
$ Until the end
This works, however it allows repeating modifiers, i.e:
This: ^/.+/[imsxe]{0,5}$
Allows this: '/blah/ii'
Allows this: '/blah/eee'
Allows this: '/blah/eise'
etc...
When it should not.
I personally use RegexPal to test, because its free and simple.
If (in order to help me) you would like to do the same, click the link above (or visit http://regexpal.com), paste my expression in the top text box
^/.+/[imsxe]{0,5}$
Then paste my tests in the bottom textbox
/^[0-9]+$/i
/^[0-9]+$/m
/^[0-9]+$/s
/^[0-9]+$/x
/^[0-9]+$/e
/^[0-9]+$/ii
/^[0-9]+$/mm
/^[0-9]+$/ss
/^[0-9]+$/xx
/^[0-9]+$/ee
/^[0-9]+$/iei
/^[0-9]+$/mim
/^[0-9]+$/sis
/^[0-9]+$/xix
/^[0-9]+$/eie
ensure you click the second checkbox at the top where it says '^$ match at line breaks (m)' to enable the multi-line testing.
Thanks for the help
Edit
After reading comments about Regex often having different delimiters i.e
/[0-9]+/ == #[0-9]+#
This is not a problem and can be factored in to my regex solution.
All I really need to know is how to prevent duplicate characters!
Edit
This bit isn't so important but it provides context
The need for such a feature is simple...
I'm using jQuery UI MultiSelect Widget written by Eric Hynds.
Simple demo found here
Now In my application, I'm extending the plugin so that certain options popup a little menu on the right when hovered. The menu that pops up can be ANY html element.
I wanted multiple options to be able to show the same element. So my API works like this:
$('#select_element_id')
// Erics MultiSelect API
.multiselect({
// MultiSelect options
})
// My API
.multiselect_side_pane({
menus: [
{
// This means, when an option with value 'MENU_1' is hovered,
// the element '#my_menu_1' will be shown. This makes attaching
// menus to options REALLY SIMPLE
menu_element: $('#my_menu_1'),
target: ['MENU_1']
},
// However, lets say we have option value 'USER_ID_132', I need the
// target name to be dynamic. What better way to be dynamic than regex?
{
menu_element: $('#user_details_box'),
targets: ['USER_FORM', '/^USER_ID_[0-9]+$/'],
onOpen: function(target)
{
// here the TARGET can be interrogated, and the correct
// user info can be displayed
// Target will be 'USER_FORM' or 'USER_ID_3' or 'USER_ID_234'
// so if it is USER_FORM I can clear the form ready for input,
// and if the target starts with 'USER_ID_', I can parse out
// the user id, and display the correct user info!
}
}
]
});
So as you can see, The whole reason I need to know if a string a regex, is so in the widget code, I can decide whether to treat the TARGET as a string (i.e. 'USER_FORM') or to treat the TARGET as an expression (i.e '/^USER_ID_[0-9]+$/' for USER_ID_234')

Answer 1

Unfortunately, the regexp string can be "anything". The forward slashes you talk about can be a lot of characters. i.e. a hash (#) will also work.
Secondly, to match up to 5 characters without having them double could probably be done with lookahead / lookbehind etc, but will create such complex regexp that it's faster to post-process it.
It is possibly faster to search for the regular expression functions (preg_match, preg_replace etc.) in code to be able to deduct where regular expressions are used.
$var = '#placeholder#';
Is a valid regular expression in PHP, but doesn't have to be one, where:
const ESCAPECHAR = '#';
$var = 'text';
$regexp = ESCAPECHAR . $var . ESCAPECHAR;
Is also valid, but might not be seen as such.

Answer 2

In order to prevent duplicate in modifier section, I'd do:
^/.+/(?:(?=[^i]*i[^i]*)?(?=[^m]*m[^m]*)?(?=[^s]*s[^s]*)?(?=[^x]*x[^x]*)?(?=[^e]*e[^e]*)?)?$