[SOLVED] sed replace positional match of unknown string divided by user-defined separator

Issue

Want to rename the (known) 3th folder within a (unknown) file path from a string, when positioned on 3th level while separator is /

Need a one-liner explicitly for sed. Because I later want use it for tar --transform=EXPRESSION

string="/db/foo/db/bar/db/folder"
echo "$string" | sed 's,db,databases,'

sed replace "db" only on 3th level

expected result

/db/foo/databases/bar/db/folder

Solution

You could use a capturing group to capture /db/foo/ and then match db. Then use use the first caputring group in the replacement using \1:

string="/db/foo/db/bar/db/folder"
echo -e "$string" | sed 's,^\(/[^/]*/[^/]*/\)db,\1databases,'

About the pattern

• ^ Start of string
• \( Start capture group
  • /[^/]*/[^/]*/ Match the first 2 parts using a negated character class
• \) Close capture group
• db Match literally

That will give you

/db/foo/databases/bar/db/folder

Answered By - The fourth bird