regex - Oracle regexp cant seem to get it right -

i have values like

cr-123456 ecr-12345 bcy-499494 134-abc ecw-ecr1233 cr-123344 

i want match lines not start ecr , regex doing ^((?!ecr)\w+) seems want. want replace matched values not begin ecr , replace them ecr , blanked because following doesn't seem work

select regexp_replace('cr-123344','^((?!ecr)\w+)','ecr') dual

any ideas have gone wrong ? want result be

ecr-123456 ecr-12345 ecr-499494 ecr-abc ecr-ecr1233 ecr-123344 

i use substring , instr replace before dash, here answer using regexp:

 aset      (select 'cr-123456'            dual          union          select 'bcy-12345'            dual          union          select 'ecr-499494'            dual          union          select '134-abc'            dual          union          select 'ecw-ecr1233'            dual          union          select 'cr-123344'            dual) select a, regexp_replace(a, '^([^-]*)','ecr') b   aset; 

results in

    a,b cr-123456,ecr-123456 bcy-12345,ecr-12345 ecr-499494,ecr-499494 134-abc,ecr-abc ecw-ecr1233,ecr-ecr1233 cr-123344,ecr-123344