SED back references

[shivarajM]

Hi,
what r back refferences in SED..........i.e., \1 \2 \3......\9
Why do we use these... please explain with an example...

Thanks

[meton_magis]

back references are useful if you want to match a regular expresion, and replace it with different data that still contains part of what was matched.

I.E. If I use regular expresion to match a html link
<a href="http://www.google.com"> search engine </a>

but I want to strip the html tags, and create a plaintext file with the URL and whatever text was linked, the 'search engine' in this example, I could
cat somefile | sed 's!<a href="(.*)">(.*)</a>!\1::\2!g'

and create a file with 2 field seperated by `::` with the previously mentioned data.