== Why is this an issue?

Overly complicated regular expressions are hard to read and to maintain and can easily cause hard-to-find bugs. If a regex is too complicated, you should consider replacing it or parts of it with regular code or splitting it apart into multiple patterns at least.


The complexity of a regular expression is determined as follows:


Each of the following operators increases the complexity by an amount equal to the current nesting level and also increases the current nesting level by one for its arguments:

* ``++|++`` - when multiple ``++|++`` operators are used together, the subsequent ones only increase the complexity by 1
* Quantifiers (``++*++``, ``+``, ``++?++``, ``++{n,m}++``, ``++{n,}++`` or ``++{n}++``)
* Non-capturing groups that set flags (such as ``++(?i:some_pattern)++`` or ``++(?i)some_pattern++``)
* Lookahead and lookbehind assertions

Additionally, each use of the following features increase the complexity by 1 regardless of nesting:


* character classes
* back references


=== Noncompliant code example

[source,python]
----
p = re.compile(r"^(?:(?:31(\/|-|\.)(?:0?[13578]|1[02]))\1|(?:(?:29|30)(\/|-|\.)(?:0?[13-9]|1[0-2])\2))(?:(?:1[6-9]|[2-9]\d)?\d{2})$|^(?:29(\/|-|\.)0?2\3(?:(?:(?:1[6-9]|[2-9]\d)?(?:0[48]|[2468][048]|[13579][26])|(?:(?:16|[2468][048]|[3579][26])00))))$|^(?:0?[1-9]|1\d|2[0-8])(\/|-|\.)(?:(?:0?[1-9])|(?:1[0-2]))\4(?:(?:1[6-9]|[2-9]\d)?\d{2})$")

if p.match($dateString):
    handleDate($dateString)
----


=== Compliant solution

[source,python]
----
p = re.compile("^\d{1,2}([-/.])\d{1,2}\1\d{1,4}$")
if p.match($dateString):
    dateParts = re.split(r"[-/.]", dateString)
    day = intval(dateParts[0])
    month = intval(dateParts[1])
    year = intval($dateParts[2])
    // Put logic to validate and process the date based on its integer parts here
----

ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

include::../message.adoc[]

include::../parameters.adoc[]

include::../highlighting.adoc[]

endif::env-github,rspecator-view[]