Replace each pattern in regexp

Question

I'm having some trouble to find the right pattern to get the string I want.

My starting string is :

,,,,C3:,D3,E3,F3,,

I would like to have

      C3:  [D3,E3,F3]

I would like to replace each starting commas by double space
Replace coma after colon by double space and left square bracket
Replace trailing commas by right square bracket

For now, I tried this :

> a = ",,,,C3:,D3,E3,F3,,"
=> ",,,,C3:,D3,E3,F3,,"
> b = a.gsub(/^,*/, "  ").gsub(/(?<=:),/, "  [").gsub(/[,]*$/,"" ).gsub(/[ ]*$/, "]")
=> "  C3:  [D3,E3,F3]"
> b == "        C3:  [D3,E3,F3]"
=> false

I can't reach to replace each starting comma by a double space to obtain 8 spaces in this case.

Could you help me to find the right regexp and if possible to improve my code, please ?

Does the s.gsub(/\G,/, ' ').sub(/,+\z/, ']').sub(/:,+/, ': [') meet your requirements? — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Jan 22, 2016 at 10:03

Wiktor Stribiżew · Accepted Answer · 2016-01-22 10:07:18Z

6

To replace each starting comma with a double space, you need to use \G operator, i.e. .gsub(/\G,/, ' '). That operator tells the regex engine to match at the start of the string and then after each successful match. So, you only replace each consecutive comma in the beginning of the string with .gsub(/\G,/, ' ').

Then, you can add other replacements:

s.gsub(/\G,/, ' ').sub(/,+\z/, ']').sub(/:,+/, ': [')

See the IDEONE demo

s = ",,,,C3:,D3,E3,F3,,"
puts s.gsub(/\G,/, '  ').sub(/,+\z/, ']').sub(/:,+/, ':  [')

Output:

        C3:  [D3,E3,F3]

answered Jan 22, 2016 at 10:07

Wiktor Stribiżew

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4 Comments

XavM Over a year ago

Thank You, I didn't know \G pattern.

Wiktor Stribiżew Over a year ago

FYI I also got rid of the lookbehind in one of your regexes, lookbehinds are rather costly regex subpatterns. Since you know the context (it is :), you can match it and then reinsert in the replacement pattern easily.

Cary Swoveland Over a year ago

An alternative is ",,,hi".gsub(/^(,+)/, ' '*'\1'.size). I am puzzled, however, as to why this works without the capture group: ",,,hi".gsub(/^,+/, ' '*'\1'.size). (I did reload IRB before testing.)

Wiktor Stribiżew Over a year ago

@CarySwoveland: It does not work the way you think: '\1'.size is always 2 because the \1 is treated as a literal string. puts '\1'.size outputs 2, it will always be two. Here is an example of how to use a "callback" with Ruby's gsub - that would work, but it would involve more coding.

Cary Swoveland · Accepted Answer · 2016-01-22 22:02:31Z

To construct the desired string, one needs to know:

the number of leading commas (the size of the string comprised of the leading commas)
the string following the leading commas up to and including the colon
the string between the comma following the colon and two or more commas

It is a simple matter to construct a regex that saves each of these three strings to a capture group:

r = /
    (,*)   # match leading commas in capture group 1
    (.+:)  # match up and including colon in capture group 2
    ,      # match comma     
    (.+)   # match any number of any characters in capture group 3
    ,,     # match two commas
    /x     # extended/free-spacing regex definition mode

",,,,C3:,D3,E3,F3,," =~ r

We can now form the desired string from the contents of the three capture groups:

"#{'  '*$1.size}#{$2} [#{$3}]"
  #=> "       C3: [D3,E3,F3]"

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