Original code with output:
trace = regexp(fileread('trace.txt'),'^(\d+)\.?\d*:[^:]+:\s+([^\n]+)','tokens','lineanchors');
trace = vertcat(trace{:})
trace = 29×2 cell array
{'0' } {'init-hold←' }
{'0' } {'pre-leak-test←' }
{'10' } {'leak-test←' }
{'3034' } {'evac←' }
{'3094' } {'pre-ramp←' }
{'3154' } {'ramp←' }
{'3180' } {'hold←' }
{'28381' } {'hold-end, theta1 = 2.89537933246 +/- 0.00364583333333 hr, theta2 = 6.98260416667 +/- 0.00364583333333 hr←'}
{'28381' } {'evac←' }
{'28681' } {'pre-ramp←' }
{'28741' } {'ramp←' }
{'28764' } {'hold←' }
{'89964' } {'hold-end, theta1 = 2.92489959614 +/- 0.003125 hr, theta2 = 13.2959027778 +/- 0.003125 hr←' }
{'89964' } {'evac←' }
{'90264' } {'pre-ramp←' }
{'90324' } {'ramp←' }
{'90347' } {'hold←' }
{'151548'} {'hold-end, theta1 = 2.47468712338 +/- 0.00319444444444 hr, theta2 = 12.7754861111 +/- 0.00319444444444 hr←'}
{'151548'} {'evac←' }
{'151848'} {'pre-ramp←' }
{'151908'} {'ramp←' }
{'151931'} {'hold←' }
{'213131'} {'hold-end, theta1 = 2.24675072007 +/- 0.00319444444444 hr, theta2 = 13.2186111111 +/- 0.00319444444444 hr←'}
{'213131'} {'evac←' }
{'213432'} {'pre-ramp←' }
{'213492'} {'ramp←' }
{'213514'} {'hold←' }
{'258874'} {'hold-end, theta1 = 2.2511608259 +/- 0.00305555555556 hr, theta2 = 11.2265972222 +/- 0.00305555555556 hr←' }
{'258874'} {'done←' }
To make the second token on each line stop at any comma (so as to remove the text that occurs after the comma behind "hold-end"):
trace = regexp(fileread('trace.txt'),'^(\d+)\.?\d*:[^:]+:\s+([^\n,]+)','tokens','lineanchors');
trace = vertcat(trace{:})
trace = 29×2 cell array
{'0' } {'init-hold←' }
{'0' } {'pre-leak-test←'}
{'10' } {'leak-test←' }
{'3034' } {'evac←' }
{'3094' } {'pre-ramp←' }
{'3154' } {'ramp←' }
{'3180' } {'hold←' }
{'28381' } {'hold-end' }
{'28381' } {'evac←' }
{'28681' } {'pre-ramp←' }
{'28741' } {'ramp←' }
{'28764' } {'hold←' }
{'89964' } {'hold-end' }
{'89964' } {'evac←' }
{'90264' } {'pre-ramp←' }
{'90324' } {'ramp←' }
{'90347' } {'hold←' }
{'151548'} {'hold-end' }
{'151548'} {'evac←' }
{'151848'} {'pre-ramp←' }
{'151908'} {'ramp←' }
{'151931'} {'hold←' }
{'213131'} {'hold-end' }
{'213131'} {'evac←' }
{'213432'} {'pre-ramp←' }
{'213492'} {'ramp←' }
{'213514'} {'hold←' }
{'258874'} {'hold-end' }
{'258874'} {'done←' }
To also avoid including the carriage returns in the second token:
trace = regexp(fileread('trace.txt'),'^(\d+)\.?\d*:[^:]+:\s+([^\n\r,]+)','tokens','lineanchors');
trace = vertcat(trace{:})
trace = 29×2 cell array
{'0' } {'init-hold' }
{'0' } {'pre-leak-test'}
{'10' } {'leak-test' }
{'3034' } {'evac' }
{'3094' } {'pre-ramp' }
{'3154' } {'ramp' }
{'3180' } {'hold' }
{'28381' } {'hold-end' }
{'28381' } {'evac' }
{'28681' } {'pre-ramp' }
{'28741' } {'ramp' }
{'28764' } {'hold' }
{'89964' } {'hold-end' }
{'89964' } {'evac' }
{'90264' } {'pre-ramp' }
{'90324' } {'ramp' }
{'90347' } {'hold' }
{'151548'} {'hold-end' }
{'151548'} {'evac' }
{'151848'} {'pre-ramp' }
{'151908'} {'ramp' }
{'151931'} {'hold' }
{'213131'} {'hold-end' }
{'213131'} {'evac' }
{'213432'} {'pre-ramp' }
{'213492'} {'ramp' }
{'213514'} {'hold' }
{'258874'} {'hold-end' }
{'258874'} {'done' }
To also include the decimal point and the digits to the right of it in the first token:
trace = regexp(fileread('trace.txt'),'^(\d+\.?\d*):[^:]+:\s+([^\n\r,]+)','tokens','lineanchors');
trace = vertcat(trace{:})
trace = 29×2 cell array
{'0.0' } {'init-hold' }
{'0.25' } {'pre-leak-test'}
{'10.5' } {'leak-test' }
{'3034.0' } {'evac' }
{'3094.25' } {'pre-ramp' }
{'3154.5' } {'ramp' }
{'3180.75' } {'hold' }
{'28381.0' } {'hold-end' }
{'28381.0' } {'evac' }
{'28681.25' } {'pre-ramp' }
{'28741.5' } {'ramp' }
{'28764.0' } {'hold' }
{'89964.25' } {'hold-end' }
{'89964.25' } {'evac' }
{'90264.5' } {'pre-ramp' }
{'90324.75' } {'ramp' }
{'90347.75' } {'hold' }
{'151548.0' } {'hold-end' }
{'151548.0' } {'evac' }
{'151848.25'} {'pre-ramp' }
{'151908.5' } {'ramp' }
{'151931.5' } {'hold' }
{'213131.75'} {'hold-end' }
{'213131.75'} {'evac' }
{'213432.0' } {'pre-ramp' }
{'213492.25'} {'ramp' }
{'213514.25'} {'hold' }
{'258874.5' } {'hold-end' }
{'258874.5' } {'done' }