Regular Expressions 101

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/
\[(\w+):?(\w+)?\]
/
g
\[ matches the character [ with index 9110 (5B16 or 1338) literally (case sensitive)
1st Capturing Group
(\w+)
\w
matches any word character (equivalent to [a-zA-Z0-9_])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
:
matches the character : with index 5810 (3A16 or 728) literally (case sensitive)
2nd Capturing Group
(\w+)?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
\w
matches any word character (equivalent to [a-zA-Z0-9_])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\] matches the character ] with index 9310 (5D16 or 1358) literally (case sensitive)
Global pattern flags
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.

Regular Expression
No Match

/
/
g

Test String