# Regular Expressions 101

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## Flavor

• PCRE2 (PHP >=7.3)
• PCRE (PHP <7.3)
• ECMAScript (JavaScript)
• Python 2.7
• Golang
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## Tools

/
(?=((?=(\3|^))(\2x)((?<=(?=z|^\3\$|(?4)).)))+)(\3x)\5\$
/
gm
(?=((?=(\3|^))(\2x)((?<=(?=z|^\3\$|(?4)).)))+)
Assert that the Regex below matches
1st Capturing Group
((?=(\3|^))(\2x)((?<=(?=z|^\3\$|(?4)).)))+
+ 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
(?=(\3|^))
Assert that the Regex below matches
2nd Capturing Group
(\3|^)
1st Alternative
\3
\3 matches the same text as most recently matched by the 3rd capturing group
2nd Alternative
^
3rd Capturing Group
(\2x)
\2 matches the same text as most recently matched by the 2nd capturing group
x
matches the character x literally (case sensitive)
4th Capturing Group
((?<=(?=z|^\3\$|(?4)).))
Positive Lookbehind
(?<=(?=z|^\3\$|(?4)).)
Assert that the Regex below matches
5th Capturing Group
(\3x)
\3 matches the same text as most recently matched by the 3rd capturing group
x
matches the character x literally (case sensitive)
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\5
matches the same text as most recently matched by the 5th capturing group
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\$ 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