How to Restrict Character Length in Regular Expressions?

Restricting Character Length in Regular Expressions

When using regular expressions to validate input, it may be necessary to limit the length of characters in the match. This can be achieved using anchors and zero-width assertions.

The example provided in the question uses a regular expression to match strings containing only letters, numbers, and certain special characters, with no character length restriction. However, when attempting to restrict the length to 15 characters using the quantifier {1,15}, a syntax error is encountered.

The reason for this is that quantifiers cannot be applied to anchors. To restrict the character length, an alternative approach is to use a lookahead anchored at the beginning of the string.

The modified expression:

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

This expression includes a lookahead, (?=.{1,15}$), which checks for a sequence of 1 to 15 characters at the end of the string. The main part of the expression compares the length of the input string.

Similarly, expressions can be crafted for various programming languages:

Flavor	Expression
ECMAScript, C	^(?=.{1,15}$)[a-zA-Z0-9]*[^$%^&amp;*;:,<>?()"]*$
Other regex flavors, Python	A(?=.{1,15}z)[a-zA-Z0-9]*[^$%^&*;:,<>?()"]*z

Additional Considerations

• Use [a-zA-Z0-9] instead of (a-z|A-Z|0-9) for efficiency.
• To allow for newlines in the input, replace {1,15} with [sS]{1,15}.

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