Regular Expressions 101

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  • Unit Tests (7)
/
^(?=^\w{3,20}$)[a-z0-9]+_?[a-z0-9]+$
/
i
^ asserts position at start of the string
Positive Lookahead
(?=^\w{3,20}$)
Assert that the Regex below matches
^ asserts position at start of the string
\w
matches any word character (equivalent to [a-zA-Z0-9_])
{3,20} matches the previous token between 3 and 20 times, as many times as possible, giving back as needed (greedy)
$ asserts position at the end of the string
Match a single character present in the list below
[a-z0-9]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
a-z matches a single character in the range between a (index 97) and z (index 122) (case insensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case insensitive)
_
matches the character _ with index 9510 (5F16 or 1378) literally (case insensitive)
Match a single character present in the list below
[a-z0-9]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
a-z matches a single character in the range between a (index 97) and z (index 122) (case insensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case insensitive)
$ asserts position at the end of the string
Global pattern flags
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
Your regular expression does not match the subject string.

Regular Expression
No Match

/
/
i

Test String