/usr/xpg4/bin/awk 'c-->0;$0~s{if(b)for(c=b+1;c>1;c--)print r[(NR-c+1)%b];print;c=a}b{r[NR%b]=$0}' b=5 a=5 s="ERROR" file

stupid
remove

keep it
*
MATCH
keep it as well



remove

important
                                                     *
MATCH at line 2:
needed
also

this line should be kept
                                                     *
MATCH at line 2:
this to save
very important



remove

keep it
*
MATCH
keep it as well

nonsense
another nonsense

keep it
keep it as well
---
important
needed
also
---
this line should be kept
this to save
very important
---
keep it
keep it as well