40 lines
1.0 KiB
Plaintext
40 lines
1.0 KiB
Plaintext
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When a back reference in a regex refers to a capturing group that hasn't been defined yet (or at all), it can never be matched and will fail with an `re.error` exception
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== Noncompliant Code Example
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[source,python]
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----
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import re
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pattern1 = re.compile(r"\1(.)") # Noncompliant, group 1 is defined after the back reference
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pattern2 = re.compile(r"(.)\2") # Noncompliant, group 2 isn't defined at all
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pattern3 = re.compile(r"(.)|\1") # Noncompliant, group 1 and the back reference are in different branches
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pattern4 = re.compile(r"(?P<x>.)|(?P=x)") # Noncompliant, group x and the back reference are in different branches
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----
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== Compliant Solution
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[source,python]
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----
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import re
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pattern1 = re.compile(r"(.)\1")
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pattern2 = re.compile(r"(?P<x>.)(?P=x)")
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----
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ifdef::env-github,rspecator-view[]
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'''
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== Implementation Specification
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(visible only on this page)
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include::../message.adoc[]
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include::../highlighting.adoc[]
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'''
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== Comments And Links
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(visible only on this page)
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include::../comments-and-links.adoc[]
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endif::env-github,rspecator-view[]
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