[SOLVED] replace an exact match for a double underscore with a string using sed

Issue

I am trying to replace an exact match for a double underscore with a string.

sed -i 's/\<__\>/.abc.def__/g' file

But this leaves the file unchanged. Grateful for any pointers. follow up from Sed match exact

Solution

If you have no overlapping matches (and your provided input has none), a sed like this will do:

sed -E  's/([^_]|^)__([^_]|$)/\1.abc.def__\2/g' file > newfile

Here, ([^_]|^)__([^_]|$) matches and captures into Group 1 (\1) any char other than _ or start of string (([^_]|^)), then matches __, and then captures into Group 2 (\2) any char other than _ or end of string (([^_]|$)).

If there can be overlapping matches, sed becomes rather difficult to use here. A perfect alternative would be using

perl -pe 's/(?<!_)__(?!_)/.abc.def__/g' file > newfile
perl -i -pe 's/(?<!_)__(?!_)/.abc.def__/g' file

The (?<!_)__(?!_) regex contains two lookarounds, (?<!_) negative lookbehind that makes sure there is no _ char immediately to the left of the current location, and (?!_) negative lookahead makes sure there is no _ char immediately to the right of the current location.

See the online demo:

#!/bin/bash
s='AFM_7499_190512_110136_001_p_EQ4H_1_s60_0012__386___Day_'

sed -E  's/([^_]|^)__([^_]|$)/\1.abc.def__\2/g' <<< "$s"
# => AFM_7499_190512_110136_001_p_EQ4H_1_s60_0012.abc.def__386___Day_
perl -i -pe 's/(?<!_)__(?!_)/.abc.def__/g' <<< "$s"
# => AFM_7499_190512_110136_001_p_EQ4H_1_s60_0012.abc.def__386___Day_

Answered By - Wiktor Stribiżew

Answer Checked By - Cary Denson (WPSolving Admin)