Regular Expressions 101

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/
(["'])\s*(SELECT)(?:\s+(?:(?!\1).)*)?\s+(FROM)(?:\s+(?:(?!\1).)*)?\1(?:[^\w]|$)
/
gm
1st Capturing Group
(["'])
Match a single character present in the list below
["']
"' matches a single character in the list "' (case sensitive)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
2nd Capturing Group
(SELECT)
SELECT matches the characters SELECT literally (case sensitive)
Non-capturing group
(?:\s+(?:(?!\1).)*)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v ])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
Non-capturing group
(?:(?!\1).)*
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Negative Lookahead
(?!\1)
Assert that the Regex below does not match
. matches any character (except for line terminators)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v ])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
3rd Capturing Group
(FROM)
FROM matches the characters FROM literally (case sensitive)
Non-capturing group
(?:\s+(?:(?!\1).)*)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v ])
Non-capturing group
(?:(?!\1).)*
\1 matches the same text as most recently matched by the 1st capturing group
Non-capturing group
(?:[^\w]|$)
1st Alternative
[^\w]
2nd Alternative
$
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