php: better way to split string into associative array

The important thing to do in maintaining accuracy is to ensure that "keys" are properly matched.

Key strings will never contain a space or an equals sign. Value strings may contain either. Value strings will run to the end of the string or be followed by a space then the next key (which may not have any spaces or equal signs).

The key string can be "greedily" matched before the occurrence of the first encountered =.

The value string must not be greedily matched. This ensures that the value is not over-extended into the next key-value pair.

The lookahead after the value string ensures that the potential following key is not damaged/consumed.

Pattern Breakdown:

([^=]+)      #capture one ore more non-equals sign (greedily) and store as capture group #1
=            #match but do not capture an equals sign
(.+?)        #capture one or more of any non-newline character (giving back when possible / non-greedy) and store as capture group #2
(?=          #start lookahead
  $          #match the end of the string
  |          #OR operator
   [^ =]+=   #match space, then one or more non-space and non-equals characters, then match equals sign
)            #end lookahead

Code: (Demo)

$msg = "ALARM_ID/I4=1010001 ALARM_STATE/U4=eventcode ALARM_TEXT/A=WMR_MAP_EXPORT LOTS/A[1]=[ STEFANO ] ALARM_STATE/U1=1 WAFER/U4=1 VI_KLARF_MAP/A=/test/klarf.map KLARF_STEPID/A=StepID KLARF_DEVICEID/A=DeviceID KLARF_EQUIPMENTID/A=EquipmentID KLARF_SETUP_ID/A=SetupID RULE_ID/U4=1234 RULE_FORMULA_EXPRESSION/A=a < b && c > d RULE_FORMULA_TEXT/A=1 < 0 && 2 > 3 RULE_FORMULA_RESULT/A=FAIL TIMESTAMP/A=10-Nov-2020 09:10:11 99999999";

preg_match_all('~([^=]+)=(.+?)(?=$| [^ =]+=)~', $msg, $out);
var_export(array_combine($out[1], $out[2]));

Output:

array (
  'ALARM_ID/I4' => '1010001',
  'ALARM_STATE/U4' => 'eventcode',
  'ALARM_TEXT/A' => 'WMR_MAP_EXPORT',
  'LOTS/A[1]' => '[ STEFANO ]',
  'ALARM_STATE/U1' => '1',
  'WAFER/U4' => '1',
  'VI_KLARF_MAP/A' => '/test/klarf.map',
  'KLARF_STEPID/A' => 'StepID',
  'KLARF_DEVICEID/A' => 'DeviceID',
  'KLARF_EQUIPMENTID/A' => 'EquipmentID',
  'KLARF_SETUP_ID/A' => 'SetupID',
  'RULE_ID/U4' => '1234',
  'RULE_FORMULA_EXPRESSION/A' => 'a < b && c > d',
  'RULE_FORMULA_TEXT/A' => '1 < 0 && 2 > 3',
  'RULE_FORMULA_RESULT/A' => 'FAIL',
  'TIMESTAMP/A' => '10-Nov-2020 09:10:11 99999999',
)

I managed this code, using basic PHP functions. I think that a regular expression makes the code more difficult to read. Most of the time, even at the expense of having more verbose code, you are better off not using regular expressions. There might also be a performance impact.

$message = "ALARM_ID/I4=1010001 ALARM_STATE/U4=eventcode ALARM_TEXT/A=WMR_MAP_EXPORT LOTS/A[1]=[ STEFANO ] ALARM_STATE/U1=1 WAFER/U4=1 VI_KLARF_MAP/A=/test/klarf.map KLARF_STEPID/A=StepID KLARF_DEVICEID/A=DeviceID KLARF_EQUIPMENTID/A=EquipmentID KLARF_SETUP_ID/A=SetupID RULE_ID/U4=1234 RULE_FORMULA_EXPRESSION/A=a < b && c > d RULE_FORMULA_TEXT/A=1 < 0 && 2 > 3 RULE_FORMULA_RESULT/A=FAIL TIMESTAMP/A=10-Nov-2020 09:10:11 99999999";

foreach (explode(' ', $message) as $word) {
    if (strpos($word, '=')) {
        if (isset($key)) $parameters[$key] = $value; 
        list($key, $value) = explode('=', $word);
    }
    else $value .= " $word";
}    
$parameters[$key] = $value;     

echo '<pre>';
print_r($parameters);
echo '</pre>';

I chose to split on the spaces, then I look for the = characters to find the words with the keys in them.

There are, of course, other ways of doing the same, but all will involve a bit of extra work because of the strange format of the message.

This routine currently does not tolerate errors in the message string, but it can easily be expanded to tolerate various types of input errors.