How to replace specific lines with regex?

Question

Suppose I have a file that contains:

Time: 60 minutes
Seconds: 3600
Milliseconds: 36000

I would like to replace any word with seconds.

%s/seconds/time/g

What am looking for is after the replace,

Time: 60 minutes
Time: 3600
MilliTime: 36000

Basically, I would like to know how I can be more specific about only replacing lines that have Seconds in the string, and avoiding the others if there were more than just these 3 lines.

Answer 1

Swapping via regular expressions is performed by the ex command

:{range}s/pattern/string/{flags} {count}

If you wish to replace instances of a work on a specific line you can supply the exact line with the absolute location. To swap matches only on line 2:

:2s/Seconds/time

Or you can use a relative location using -/+. To swap only on a line two above the cursor:

:-2s/Seconds/time

Range can also be defined by using visual mode to select the area to perform the search. Visually select a region and press :

:'<,'>s/Second/time

Ranges can also be absolute or relative

:2,4s/Second/time
:-2,+2s/Second/time

The first does lines 2 through 4 and the second does 2 lines above the cursor to two lines below the cursor.

If you want to swap all cases you can supply the range and the flag g which finds all matches. Supply the % range for the entire document.

:%s/Seconds/time/g

Some commonly used flags are:

• c: Confirms each substitution (yes/no/abort/quit)
• g: Replace all occurrences in a line. Without this flag only the first instance on a line is swapped.
• i: Ignore the case of the pattern
• I: DO NOT ignore the case of the pattern

For further information check out the built in help :h :substitute.

Answer 2

Aside from jecxjo's answer, there are also 2 other ways to replace only seconds and leave milliseconds` alone (without using ranges, which may be unwieldy for large files):

1. Anchor the pattern to the start of the line with ^: %s/^seconds/time/; works in this specific case, but (obviously) won't work if you have an instance of seconds somewhere that isn't at the start of the line

2. More flexible is using a "word" match: %s/\<seconds\>/time/. The \< matches the "beginning of a word", and the \> matches the "end of a word"; so adding them both only matches if seconds is a word on it's own (and not part of a word); in this case, you could also use :%s/\<seconds/time/, because in milliseconds, seconds isn't at the start of the word.