Regular Expressions 101

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\b(\d+)\b
/
\b assert position at a word boundary: (^\w|\w$|\W\w|\w\W)
1st Capturing Group
(\d+)
\d
matches a digit (equivalent to [0-9])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\b assert position at a word boundary: (^\w|\w$|\W\w|\w\W)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/

Test String