How to Restrict Character Length in Regular Expressions Using Lookahead?

Restricting Character Length in Regular Expressions

Your initial regular expression allowed for an unlimited character length, but when you attempted to restrict it to 15 characters with the quantifier {1,15}, you encountered an error. This is because quantifiers cannot be applied to anchors, such as the ^ start-of-string anchor.

Solution Using Lookahead

To restrict the character length effectively, you can utilize a positive lookahead anchored at the beginning of the string:

^(?=.{1,15}$)[a-zA-Z0-9]*[^$%^&*;:,?()"]*

This lookahead ensures that the subsequent string matches 1 to 15 characters (specified by the quantifier {1,15}) and ends immediately after the match. The $ anchor indicates the end of the string.

Avoidance of Limiting Quantifiers

Using a quantifier at the end of the regex, such as ^[a-zA-Z0-9]*[^$%^&*;:,?()"]{1,15}$, would incorrectly restrict the length of only the second character class to 1 to 15 characters. It would not limit the length of the entire string.

How the Lookahead Works

The lookahead (?=.{1,15}$) evaluates the subsequent 1 to 15 characters, including the newline character at the end of the string (due to the $ anchor). If this condition is met, the expression returns true, otherwise false.

Handling Newline Sequences

If your input can contain newline sequences, you can replace the . wildcard with the [sS] portable any-character construct:

^(?=[\s\S]{1,15}$)[a-zA-Z0-9]*[^$%^&*;:,?()"]*

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