Append the end of one line to the start of the next with sed -

i'm looking sed solutions following:

corrupted input:

a 123 dgbsdgsbg 345 gsgsdgdgs 23 2 afaffaaf 324 fsgdggsdg 345 avsa fasf 

expected output:

a 123 dgbsdgsbg 345 gsgsdgdgs 232 afaffaaf 324 fsgdggsdg 345 avsafasf 

how can trailing a [0-9].* appended start of next line. far have:

$ sed -r 's/ (a [0-9]+.*)/\n\1/' file 123 dgbsdgsbg 345 gsgsdgdgs 23 2 afaffaaf 324 fsgdggsdg 345 avsa fasf 

this might work (gnu sed):

 sed -r '$!n;s/ (a[^\n]*)\n/\n\1/;p;d' file