Regular Expressions 101

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/
[A-Z]{2}\d{4}(?=\s\d+)
/
gm
Match a single character present in the list below
[A-Z]
{2} matches the previous token exactly 2 times
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
\d
matches a digit (equivalent to [0-9])
{4} matches the previous token exactly 4 times
Positive Lookahead
(?=\s\d+)
Assert that the Regex below matches
\s matches any whitespace character (equivalent to [\r\n\t\f\v ])
\d
matches a digit (equivalent to [0-9])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
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