The ^ anchor asserts that we are at the beginning of the string. [a-z]{1} matches one lower-case letter. The {1} is unneeded. [a-z0-9_]{3,13} matches 3 to 13 chars. In case-insensitive mode, in many engines it could be replaced by \w{3,13} The $ anchor asserts that we are at the end of the string. More @Wikipedia
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