How to do conditional regex replace in PowerShell

Question

I am trying to use -replace statement to do the conditional replacement in PowerShell.

Below is my reproduceable example:

$content = @'
a&lt;a
a&gt;a
a&apos;a
a&quote;a
a&amp;a
johnson&johnson
J&J
R&D
P&AB
M&ABC
O&ABCD
B&SS-IHC
S&B
GC&E
M&G
P&D
'@

I want to replace all the & with &, but if the string is ended with ; then don't replace. The result would be:

a&lt;a
a&gt;a
a&apos;a
a&quote;a
johnson&amp;johnson
J&amp;J
R&amp;D
P&amp;AB
M&amp;ABC
O&amp;ABCD
B&amp;SS-IHC
S&amp;B
GC&amp;E
M&amp;G
P&amp;D

So far I can only come up with $modifiedContent = $content -replace '(\w)&(\w*)[^;]|$' , '$1&$2' and I am getting nowhere. How can I add a condition correctly that will do the trick?

Thanks in advance.

You can match & not followed by any of the alternatives &(?!lt|gt|amp|quote|apos) regex101.com/r/AYlNsW/1 — The fourth bird
– The fourth bird, Commented Nov 18, 2020 at 10:43

Wiktor Stribiżew · Accepted Answer · 2020-11-18 10:44:27Z

2

You may use

\b&\b(?!\w+;)

Replace with &. See a regex demo.

Details

\b - a word char is required immediately to the left of the current location
& - a & char
\b - a word char is required immediately to the right of the current location
(?!\w+;) - immediately to the right, there should be no 1+ word chars and a ;.

answered Nov 18, 2020 at 10:44

Wiktor Stribiżew

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The fourth bird · Accepted Answer · 2020-11-18 10:54:28Z

1

You could also specify what you don't want to match as it seems you want to exclude certain special entities.

Use a negative lookahead with an alternation to assert what is at the right is not any of the listed alternatives followed by a ; and replace with &

&(?!(?:lt|gt|amp|quote|apos);)

Explanation

& Match literally
(?! Negative lookahead
- (?:lt|gt|amp|quote|apos); Match any of the listed followed by ;
) Close lookahead

Regex demo

edited Nov 18, 2020 at 10:54

answered Nov 18, 2020 at 10:49

The fourth bird

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Алексей Семенов · Accepted Answer · 2020-11-18 12:49:13Z

1

([regex]"(\S*)(&)(?!\S*;)(\S*)").Replace($content, '$1&amp;$3')

answered Nov 18, 2020 at 12:49

Алексей Семенов

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