php - Preg_replace not working as wanted -

basically have following text stored in $text var :

$text = 'an airplane accelerates down runway @ 3.20 m/s2 32.8 s until lifts off ground. determine distance traveled before takeoff'. 

i have function replaces keywords on text array named $replacements (i did var_dump on it) :

'm' => string 'meter' (length=5) 'meters' => string 'meter' (length=5) 's' => string 'second' (length=6) 'seconds' => string 'second' (length=6) 'n' => string 'newton' (length=6) 'newtons' => string 'newton' (length=6) 'v' => string 'volt' (length=4) 'speed' => string 'velocity' (length=8) '\/' => string 'per' (length=3) 's2' => string 'secondsquare' (length=12) 

the text goes through following function :

$toreplace = array_keys($replacements);  foreach ($toreplace $r){     $text = preg_replace("/\b$r\b/u", $replacements[$r], $text); } 

however, there difference between expect , output :

expected output : airplane accelerates down runway @ 3.20 meterpersecondsquare 32.8 second until lifts off ground determine distance traveled before takeoff   function output : airplane accelerates down runway @ 3.20 meterpers2 32.8 second until lifts off ground determine distance traveled before takeoff  

notice expect 'meterpersecondsquare' , 'meterpers2' (the 's2' isn't replaced) while 'm' , '/' replaced values.

i noticed when put m/s instead of m/s2 works fine , gives :

an airplane accelerates down runway @ 3.20 meterpersecond 32.8 second until lifts off ground determine distance traveled before takeoff  

so problem doesn't match s2. thoughts why case?

move s2 replacement before s replacement.

since doing replacement 1 @ time, destroying s2 before gets chance replace it.

3.20 m/s2 transformed this

[m] 3.20 meter/s2

[s] 3.20 meter/second2

[/] 3.20 meterpersecond2

which results in meterpersecond2

here proper order

'm' => string 'meter' (length=5) 'meters' => string 'meter' (length=5) 's2' => string 'secondsquare' (length=12) 's' => string 'second' (length=6) 'seconds' => string 'second' (length=6) 'n' => string 'newton' (length=6) 'newtons' => string 'newton' (length=6) 'v' => string 'volt' (length=4) 'speed' => string 'velocity' (length=8) '\/' => string 'per' (length=3)