Make PROCESSING_INSTRUCTION regexp lazy

[kddnewton]

Otherwise this fails with input like

<list><value id="all"><label>All</label></value><value id="one-one"><?group one?><label>One</label></value><value id="one-two"><?group one?><label>Two</label></value><value id="two-one"><?group two?><label>One</label></value><value id="two-two"><?group two?><label>Two</label></value></list>

where it will go to the end of the line to find the ?>

[cla-assistant]

All committers have signed the CLA.

[kddnewton]

@bd82 any chance you can take a look?