Regular Expressions 101

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/
(?<=\d\-\d\-\d|\d.\d|\d.\d.\d.\d|\d.\d.\d).(?=\d)
/
gm
Positive Lookbehind
(?<=\d\-\d\-\d|\d.\d|\d.\d.\d.\d|\d.\d.\d)
Assert that the Regex below matches
1st Alternative
\d\-\d\-\d
\d
matches a digit (equivalent to [0-9])
{4} matches the previous token exactly 4 times
\- matches the character - literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{2} matches the previous token exactly 2 times
\- matches the character - literally (case sensitive)
\d
matches a digit (equivalent to [0-9])
{2} matches the previous token exactly 2 times
2nd Alternative
\d.\d
\d
matches a digit (equivalent to [0-9])
{4} matches the previous token exactly 4 times
. matches any character (except for line terminators)
\d
matches a digit (equivalent to [0-9])
{4} matches the previous token exactly 4 times
3rd Alternative
\d.\d.\d.\d
\d
matches a digit (equivalent to [0-9])
{4} matches the previous token exactly 4 times
. matches any character (except for line terminators)
\d
matches a digit (equivalent to [0-9])
{1} matches the previous token exactly one time (meaningless quantifier)
. matches any character (except for line terminators)
\d
matches a digit (equivalent to [0-9])
{1} matches the previous token exactly one time (meaningless quantifier)
. matches any character (except for line terminators)
\d
matches a digit (equivalent to [0-9])
4th Alternative
\d.\d.\d
\d
matches a digit (equivalent to [0-9])
{4} matches the previous token exactly 4 times
. matches any character (except for line terminators)
\d
matches a digit (equivalent to [0-9])
{2} matches the previous token exactly 2 times
. matches any character (except for line terminators)
\d
matches a digit (equivalent to [0-9])
.
matches any character (except for line terminators)
*? matches the previous token between zero and unlimited times, as few times as possible, expanding as needed (lazy)
Positive Lookahead
(?=\d)
Assert that the Regex below matches
\d matches a digit (equivalent to [0-9])
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)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String