#Japt, (削除) 51 (削除ここまで)(削除) 48 (削除ここまで)(削除) 45 (削除ここまで)(削除) 36 (削除ここまで)(削除) 33 (削除ここまで) 19 bytes

Japt, (削除) 51 (削除ここまで)(削除) 48 (削除ここまで)(削除) 45 (削除ここまで)(削除) 36 (削除ここまで)(削除) 33 (削除ここまで) 19 bytes

Here's a basic demonstration of how and why this works (using . as the filler char):

Here's a basic demonstration of how and why this works (using . in place of a space):

Here's a basic demonstration of how this works:

antant // Starting with "antant" and replacing with three spaces,
 ant // this is the next step. However, the three spaces at the beginning are
false // now replaced, ending the recursive replacement.
antant // Replacing with two spaces, however, there's no chance for it to
 ant // match three spaces in the next step, so it matches "boa" at the beginning,
 nt // then "ant" once more here.
 // All that's left now is three spaces, which becomes two spaces, then stops.
true // Now the string is only spaces, so true is returned.

Here's a basic demonstration of how and why this works (using . as the filler char):

First match at the end: 
eleant
ele.. (ant)
el.. (eel)
... (elk)
.. (...)
true
First match at the beginning: 
antmua
..mua (ant)
...a (emu)
..a (...)
.. (boa)
true
First match in the middle: 
cantay
c..ay (ant)
..ay (cat)
... (jay)
.. (...)
true