Regular Expressions 101

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/
(?:^|\W)([0-9]+_[a-zA-Z0-9])\w+
/
Non-capturing group
(?:^|\W)
1st Alternative
^
^ asserts position at start of the string
2nd Alternative
\W
\W matches any non-word character (equivalent to [^a-zA-Z0-9_])
1st Capturing Group
([0-9]+_[a-zA-Z0-9])
Match a single character present in the list below
[0-9]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
_ matches the character _ with index 9510 (5F16 or 1378) literally (case sensitive)
Match a single character present in the list below
[a-zA-Z0-9]
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
\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)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/

Test String