# Regular Expressions 101

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## Flavor

• PCRE2 (PHP >=7.3)
• PCRE (PHP <7.3)
• ECMAScript (JavaScript)
• Python 2.7
• Golang
• Java 8

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• Unit Tests

## Tools

/
^(?=(\{([^+{}]|(?1))\})\$)\{+(\{[^+{}]\})\}+\$
/
mg
^ asserts position at start of a line
(?=(\{([^+{}]|(?1))\})\$)
Assert that the Regex below matches
1st Capturing Group
(\{([^+{}]|(?1))\})
\{ matches the character { literally (case sensitive)
2nd Capturing Group
([^+{}]|(?1))
1st Alternative
[^+{}]
Match a single character not present in the list below
[^{}]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
{} matches a single character in the list {} (case sensitive)
2nd Alternative
(?1)
(?1) recurses the 1st subpattern
\} matches the character } literally (case sensitive)
\$ asserts position at the end of a line
\{
matches the character { literally (case sensitive)
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
3rd Capturing Group
(\{[^+{}]\})
\{ matches the character { literally (case sensitive)
Match a single character not present in the list below
[^{}]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
{} matches a single character in the list {} (case sensitive)
\} matches the character } literally (case sensitive)
\}
matches the character } literally (case sensitive)
+ 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
m modifier: multi line. Causes ^ and \$ to match the begin/end of each line (not only begin/end of string)
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

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mg