# Regular Expressions 101

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• PCRE2 (PHP >=7.3)
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## Tools

/
^((a{1,4}|a?b|ba){1,3}|(a{1,3}|b){1,4}|((a{2,3}|b){2}|aaba|abaa){2})\$
/
gm
^ asserts position at start of a line
1st Capturing Group
((a{1,4}|a?b|ba){1,3}|(a{1,3}|b){1,4}|((a{2,3}|b){2}|aaba|abaa){2})
1st Alternative
(a{1,4}|a?b|ba){1,3}
2nd Capturing Group
(a{1,4}|a?b|ba){1,3}
{1,3} matches the previous token between 1 and 3 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
a{1,4}
a
matches the character a with index 9710 (6116 or 1418) literally (case sensitive)
2nd Alternative
a?b
a
matches the character a with index 9710 (6116 or 1418) literally (case sensitive)
b matches the character b with index 9810 (6216 or 1428) literally (case sensitive)
3rd Alternative
ba
ba
matches the characters ba literally (case sensitive)
2nd Alternative
(a{1,3}|b){1,4}
3rd Capturing Group
(a{1,3}|b){1,4}
{1,4} matches the previous token between 1 and 4 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
a{1,3}
a
matches the character a with index 9710 (6116 or 1418) literally (case sensitive)
2nd Alternative
b
b matches the character b with index 9810 (6216 or 1428) literally (case sensitive)
3rd Alternative
((a{2,3}|b){2}|aaba|abaa){2}
4th Capturing Group
((a{2,3}|b){2}|aaba|abaa){2}
{2} matches the previous token exactly 2 times
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
(a{2,3}|b){2}
5th Capturing Group
(a{2,3}|b){2}
{2} matches the previous token exactly 2 times
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
a{2,3}
a
matches the character a with index 9710 (6116 or 1418) literally (case sensitive)
2nd Alternative
b
b matches the character b with index 9810 (6216 or 1428) literally (case sensitive)
2nd Alternative
aaba
aaba
matches the characters aaba literally (case sensitive)
3rd Alternative
abaa
abaa
matches the characters abaa 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.

/
/
gm