Regular Expressions 101

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Function

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  • Unit Tests (2)
/
^(([^.])|([.])(?!\3))+$
/
gm
^ asserts position at start of a line
1st Capturing Group
(([^.])|([.])(?!\3))+
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
A repeated capturing group will only capture the last iteration. Put a capturing group around the repeated group to capture all iterations or use a non-capturing group instead if you're not interested in the data
1st Alternative
([^.])
2nd Capturing Group
([^.])
Match a single character not present in the list below
[^.]
. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
2nd Alternative
([.])(?!\3)
3rd Capturing Group
([.])
Match a single character present in the list below
[.]
. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
Negative Lookahead
(?!\3)
Assert that the Regex below does not match
\3 matches the same text as most recently matched by the 3rd capturing group
$ asserts position at the end of a line
Global pattern flags
g modifier: global. All matches (don't return after first match)
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)

Regular Expression

/
/
gm

Unit Tests

given the string test.ing. assert that regex does match
given the string test..ing. assert that regex does not match