How to use Ruby gsub with regex to do partial string substitution

I have a pipe delimited file which has a line

H||CUSTCHQH2H||PHPCCIPHP|1010032000|28092017|25001853||||

I want to substitute the date (28092017) with a regex "[0-9]{8}" if the first character is "H"

I tried the following example to test my understanding where Im trying to subtitute "a" with "i".

str = "|123||a|"
str.gsub /\|(.*?)\|(.*?)\|(.*?)\|/, "\|\\1\|\|\\1\|i\|"

But this is giving o/p as "|123||123|i|"

Any clue how this can be achieved?

Solution

You may replace the first occurrence of 8 digits inside pipes if a string starts with H using

s = "H||CUSTCHQH2H||PHPCCIPHP|1010032000|28092017|25001853||||"
p s.gsub(/\A(H.*?\|)[0-9]{8}(?=\|)/, '\100000000')
# or
p s.gsub(/\AH.*?\|\K[0-9]{8}(?=\|)/, '00000000')

See the Ruby demo. Here, the value is replaced with 8 zeros.

Pattern details

\A - start of string (^ is the start of a line in Ruby)
(H.*?\|) - Capturing group 1 (you do not need it when using the variation with \K): H and then any 0+ chars as few as possible
\K - match reset operator that discards the text matched so far
[0-9]{8} - eight digits
(?=\|) - the next char must be |, but it is not added to the match value since it is a positive lookahead that does not consume text.

The \1 in the first gsub is a replacement backreference to the value in Group 1.