Restricting character length in a regular expression

by

You cannot apply quantifiers to anchors. Instead, to restrict the length of the input string, use a lookahead anchored at the beginning:

// ECMAScript (JavaScript, C++)
^(?=.{1,15}$)[a-zA-Z0-9]*[^$%^&*;:,<>?()\"']*$
^^^^^^^^^^^

// Or, in flavors other than ECMAScript and Python
\A(?=.{1,15}\z)[a-zA-Z0-9]*[^$%^&*;:,<>?()\"']*\z
^^^^^^^^^^^^^^^

// Or, in Python
\A(?=.{1,15}\Z)[a-zA-Z0-9]*[^$%^&*;:,<>?()\"']*\Z
^^^^^^^^^^^^^^^

Also, I assume you wanted to match 0 or more letters or digits with (a-z|A-Z|0-9)*. It should look like [a-zA-Z0-9]* (i.e. use a character class here).

Why not use a limiting quantifier, like {1,15}, at the end?

Quantifiers are only applied to the subpattern to the left, be it a group or a character class, or a literal symbol. Thus, ^[a-zA-Z0-9]*[^$%^&*;:,<>?()\"']{1,15}$ will effectively restrict the length of the second character class [^$%^&*;:,<>?()\"'] to 1 to 15 characters. The ^(?:[a-zA-Z0-9]*[^$%^&*;:,<>?()\"']*){1,15}$ will “restrict” the sequence of 2 subpatterns of unlimited length (as the * (and +, too) can match unlimited number of characters) to 1 to 15 times, and we still do not restrict the length of the whole input string.

How does the lookahead restriction work?

The (?=.{1,15}$) / (?=.{1,15}\z) / (?=.{1,15}\Z) positive lookahead appears right after ^/\A (note in Ruby, \A is the only anchor that matches only start of the whole string) start-of-string anchor. It is a zero-width assertion that only returns true or false after checking if its subpattern matches the subsequent characters. So, this lookahead tries to match any 1 to 15 (due to the limiting quantifier {1,15}) characters but a newline right at the end of the string (due to the $/\z/\Z anchor). If we remove the $ / \z / \Z anchor from the lookahead, the lookahead will only require the string to contain 1 to 15 characters, but the total string length can be any.

If the input string can contain a newline sequence, you should use [\s\S] portable any-character regex construct (it will work in JS and other common regex flavors):

// ECMAScript (JavaScript, C++)
^(?=[\s\S]{1,15}$)[a-zA-Z0-9]*[^$%^&*;:,<>?()\"']*$
 ^^^^^^^^^^^^^^^^^

// Or, in flavors other than ECMAScript and Python
\A(?=[\s\S]{1,15}\z)[a-zA-Z0-9]*[^$%^&*;:,<>?()\"']*\z
  ^^^^^^^^^^^^^^^^^^

// Or, in Python
\A(?=[\s\S]{1,15}\Z)[a-zA-Z0-9]*[^$%^&*;:,<>?()\"']*\Z
  ^^^^^^^^^^^^^^^^^^

More Related Contents:

  1. Restricting Character length in Regular expression
  2. Find list of strings that matches a pattern that start with a special character « and end with a special character »
  3. Need Pattern for password field?
  4. Javascript Regex for decimal [duplicate]
  5. How to append whole set of model to formdata and obtain it in MVC
  6. How to replace plain URLs with links?
  7. Regex to get string between curly braces
  8. What is the meaning of the ‘g’ flag in regular expressions?
  9. What is the difference between RegExp’s exec() function and String’s match() function?
  10. Is there a version of JavaScript’s String.indexOf() that allows for regular expressions?
  11. split string only on first instance of specified character
  12. Remove HTML Tags in Javascript with Regex
  13. javascript regex split produces too many items
  14. How to record webcam and audio using webRTC and a server-based Peer connection
  15. force browsers to get latest js and css files in asp.net application
  16. How to get possibly overlapping matches in a string
  17. JavaScript Regular Expression Email Validation [duplicate]
  18. Extract image src from a string
  19. how to extract floating numbers from strings in javascript
  20. Javascript unicode string, chinese character but no punctuation
  21. javascript regex to extract anchor text and URL from anchor tags
  22. Example of a regular expression for phone numbers
  23. Regex for parsing single key: values out of JSON in Javascript
  24. Regex to split camel case
  25. javascript split string by space, but ignore space in quotes (notice not to split by the colon too)
  26. Mask US phone number string with JavaScript
  27. JavaScript remove ZERO WIDTH SPACE (unicode 8203) from string
  28. What’s different between Python and Javascript regular expressions?
  29. Extract keyword from Google search in Javascript
  30. Regex: only alphanumeric but not if this is pure numeric

Leave a Comment