Regular Expressions 101

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  • PCRE2 (PHP >=7.3)
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/
^(19[0-9][0-9]|20[01][0-9]|2020)$
/
gm
^ asserts position at start of a line
1st Capturing Group
(19[0-9][0-9]|20[01][0-9]|2020)
1st Alternative
19[0-9][0-9]
19 matches the characters 19 literally (case sensitive)
Match a single character present in the list below
[0-9]
0-9 a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
Match a single character present in the list below
[0-9]
0-9 a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
2nd Alternative
20[01][0-9]
20 matches the characters 20 literally (case sensitive)
Match a single character present in the list below
[01]
01 matches a single character in the list 01 (case sensitive)
Match a single character present in the list below
[0-9]
0-9 a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
3rd Alternative
2020
2020 matches the characters 2020 literally (case sensitive)
$ 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)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String