[docs] [issue25517] regex howto example in "Lookahead Assertions"

[Ezio Melotti]

Changes by Ezio Melotti <ezio.melotti at gmail.com>:


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components: +Regular Expressions
nosy: +ezio.melotti, mrabarnett
versions:  -Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue25517>
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