Double match - substitute pattern on subsequent line if prevous line matches another pattern?

Question

I'm trying to replace VEVENT to VTODO entries in an .ics file if it matches current date on another line (it was exported incorrectly):

BEGIN:VCALENDAR
BEGIN:VEVENT
DTSTART:20220340T140000
END:VEVENT
BEGIN:VEVENT
DTSTART:20230620T193700
END:VEVENT
BEGIN:VEVENT
DTSTART:20210210T193800
END:VEVENT
END:VCALENDAR

The second VEVENT entry has current time so it should become:

BEGIN:VTODO
DTSTART:20230620T193700
END:VTODO

There are more entries between BEGIN:VEVENT and END:VEVENT lines, I've redacted them for clarity.

I've tried this with sed, but the ranges pick the first occurrence of VEVENT in the entire file, not first occurrence after (or before) the matched pattern, so it replaces all of them.

sed -i "/BEGIN:VEVENT/,/DTSTART:$(date +%Y%m%dT%H%M)/{s/VEVENT/VTODO/}" org.ics

I was trying to adapt it to another question here, which I thought was relevant: Find a string and replace another string after the first is found

sed -n "/DTSTART:$(date +%Y%m%dT%H%M)/,${/END:VEVENT/{x//{x b}g s/VEVENT/VTODO/}}" org.ics

but it didn't work at all: sed: -e expression #1, char 25: unexpected ,'`

Answer 1

So the following should work:

sed 'H;/BEGIN:VEVENT/h;/END:VEVENT/!d;x;/DTSTART:'"$(date +%Y%m%dT%H%M)"'/s/VEVENT/VTODO/g' org.ics

Explanation:

H: Appends to the hold space which creates kind of a buffer (space), then we do pattern matching

/BEGIN:VEVENT/h and store it in the hold space, so now we run another pattern matching

/END:VEVENT/!d and if the pattern doesn't match, then delete it.

x; Exchanges hold space with pattern space so right now we have the lines we need in the pattern space

Finally, substitute the line DTSTART... if it matches the date. So s/.../.../g is executed only if there is a match.

/DTSTART:'"$(date +%Y%m%dT%H%M)"/s/VEVENT/VTODO/g

Update

sed '1n;H;/BEGIN:VEVENT/h;/END:VEVENT/!d;x;/DTSTART:'"$(date +%Y%m%dT193700)"'/s/VEVENT/VTODO/g' org.ics | sed '$aEND:CALENDAR

Answer 2

Robustly using any awk in any shell on every Unix box:

$ cat tst.sh
#!/usr/bin/env bash

awk -v now="$(date +'%Y%m%dT%H%M')" '
    $0 == "BEGIN:VEVENT" {
        eventType = 1
        event = $0
        next
    }
    eventType {
        event = event ORS $0
        if ( $0 == ("DTSTART:" now) ) {
            eventType = 2
        }
        if ( $0 == "END:VEVENT" ) {
            if ( eventType == 2 ) {
                sub(/^BEGIN:VEVENT/,"BEGIN:VTODO",event)
                sub(/END:VEVENT$/,"END:VTODO",event)
            }
            print event
        }
        next
    }
    { print }
' "${@:--}"

$ ./tst.sh file
BEGIN:VCALENDAR
BEGIN:VEVENT
DTSTART:20220340T140000
END:VEVENT
BEGIN:VTODO
DTSTART:20230620T193700
END:VTODO
BEGIN:VEVENT
DTSTART:20210210T193800
END:VEVENT

The only way that would fail would be if you could have some other section in your input that can contain lines that match any of the BEGIN:, END:, or DSTART: lines but I doubt if that could be valid in your input - you'd have to show us how it's specified if it is.

The important thing about the above solution is that it's comparing whole literal lines to the target strings, not just doing partial matches on wherever they might occur in the input.

Answer 3

It is not solving, but the hint only:

sed s/'$'/':'/ org.ics |awk 'BEGIN{RS="BEGIN";FS=":";CD="20220340"} NR>1 {AA[1]="BEGIN";for(i=2;i<=NF;i++){AA[i]=$i;if($i =="DTSTART" && $(i+1) ~ CD ){AA[2]="VTODO"}};for(i in AA){printf AA[i]":"};print ""}'

Where sed command adds : (colon) at the end to each line, RS="BEGIN" sets the RowSeparator to keyword BEGIN (that's why it dismisses!), FS=":" sets the FieldSeparator to ":" . In each "row" (from the keyword BEGIN to the next BEGIN, better said "record") the fields are copied into the array members AA[i] starting with i=2, because the AA[1] has to be the dismissed BEGIN. While checking the keyword DTSTART and current date, and changing the VEVENT to VTODO if necessary. After the line is checked, the array AA[] is printed and with : as separator.

This is the hint only, I did not test it. You have to debug and tune up by yourself. Namely you may clear the ending : if it is unwanted.

Answer 4

We can have a solution like this in TXR:

@(repeat)
@  (cases)
BEGIN:VEVENT
DTSTART:@{date}T@{time}
END:VEVENT
@    (output)
BEGIN:VTODO
DTSTART:@{date}T@{time}
END:VTODO
@    (end)
@  (or)
@line
@    (do (put-line line))
@  (end)
@(end)

This, by itself, will rewrite all the entries to VTODO:

$ txr  cal.txr cal.ics
BEGIN:VCALENDAR
BEGIN:VTODO
DTSTART:20220340T140000
END:VTODO
BEGIN:VTODO
DTSTART:20230620T193700
END:VTODO
BEGIN:VTODO
DTSTART:20210210T193800
END:VTODO
END:VCALENDAR

We can bind the date and time variables from the command line though:

$ txr -Ddate=20230620 cal.txr cal.ics
BEGIN:VCALENDAR
BEGIN:VEVENT
DTSTART:20220340T140000
END:VEVENT
BEGIN:VTODO
DTSTART:20230620T193700
END:VTODO
BEGIN:VEVENT
DTSTART:20210210T193800
END:VEVENT
END:VCALENDAR

Now that only matches and rewrites the second entry.

Pretty version: