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[xyz]level 1.1[/xyz]
[xyz]level 1.2[/xyz]

<table><tr><td>level 1.1</td></tr></table>
<table><tr><td>level 1.2</td></tr></table>

[xyz]level 1.1 [xyz]level 2.1[/xyz] rest of 1.1 [/xyz]

<table><tr><td>level 1.1 <table><tr><td>level 2.1</td></tr></table>res
+t of 1.1 </td></tr></table>

[xyz] error [xyz] level 1.1 [xyz] level 2.1 [/xyz] [/xyz]
[xyz] level 1.2 [/xyz]

[xyz] error <table><tr><td>level 1.1 <table><tr><td>level 2.1 </td></t
+r></table> </td></tr></table>
<table><tr><td>level 1.2 </td></tr></table>

while ($string =~ s/
  \[xyz\] 
...
  /
  "<table><tr><td>$1</td></tr></table>"
  /gsiex;

    while ($string =~ s/
    (\[\Q$tag\E\])
...
    /gsiex) {
    1; # do nothing
    }