Ruby: Conditional replace in String using gsub

Question

Given an input string:

<m>1</m>
<m>2</m>
<m>10</m>
<m>11</m>

I would like to replace all values that are not equal to 1 with 5.
So the output String should look like:

<m>1</m>
<m>5</m>
<m>5</m>
<m>5</m>

I tried using:

gsub(/(<m>)([^1])(<\/m>)/, '\15\3')

But this will not replace 10 and 11.

I have to admit that I'm kind of curious why.

Jonas Elfström
– Jonas Elfström

2010-12-06 14:38:30 +00:00
Commented Dec 6, 2010 at 14:38 — Jonas Elfström
– Jonas Elfström, Commented Dec 6, 2010 at 14:38

Theo · Accepted Answer · 2010-12-06 14:27:09Z

21

#gsub can optionally take a block and will replace with the result of that block:

subject.gsub(/\d+/) { |m| m == '1' ? m : '5' }

answered Dec 6, 2010 at 14:27

Theo

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3 Comments

Martin Flucka Over a year ago

what I needed in the end was something like this: gsub(/(<m>)(\d+)(<\/m>)/) { | m1 | m1.gsub(/\d+/) { | m2 | if ['1', '2', '4'].include?(m2) m2 else '5' end } }

Theo Over a year ago

If you're using Ruby 1.9 you can use the pattern /(?<=<m>)(\d+)(?=<\/m>)/ to simplify it. The (?<=...) and (?=...) are a lookbehind and a lookahead, and those won't be included in the matched string.

Theo Over a year ago

or you can use $1, $2 and $3 in the block and just ignore the parameter: subject.gsub(/(<m>)(\d+)(<\/m>)/) { |m| $1 + ($2 == '1' ? $2 : '5') + $3 }

Jonas Elfström · Accepted Answer · 2010-12-06 14:36:54Z

4

Without regexp just because it's possible

"1 2 10 11".split.map{|n| n=='1' ? n : '5'}.join(' ')

answered Dec 6, 2010 at 14:36

Jonas Elfström

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Comments

Tim Pietzcker · Accepted Answer · 2010-12-06 15:30:51Z

3

result = subject.gsub(/\b(?!1\b)\d+/, '5')

Explanation:

\b    # match at a word boundary (in this case, at the start of a number)
(?!   # assert that it's not possible to match
 1    # 1
 \b   # if followed by a word boundary (= end of the number)
)     # end of lookahead assertion
\d+   # match any (integer) number

Edit:

If you just wish to replace numbers that are surrounded by <m> and </m> then you can use

result = subject.gsub(/<m>(?!1\b)\d+<\/m>/, '<m>5</m>')

edited Dec 6, 2010 at 15:30

answered Dec 6, 2010 at 14:24

Tim Pietzcker

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