Regular expression for 6 digits OR 25 digits

For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))

I can write a regular expression to match ANY number of digits between 6 and 25 ...

^[[:digit:]]{6,25}$

but how would I write a regular expression to match either 6 digits or 25 digits, but no number in between?

^([[:digit:]]{6}|[[:digit:]]{25})$

(The regular expressions in this example are Tcl and PHP/ereg types. use \d instead of [[:digit:]] for PHP/preg and Perl.)

You may wonder why I want this. A client with 6 digit or 25 digit part numbers, a year number that can be 2 or 4 digits ...
(written 2008-04-16)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles

T247 - Tcl/Tk - Advanced Regular Expressions
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [1195] Regular Express Primer - (2007-05-20)
  [1305] Regular expressions made easy - building from components - (2007-08-16)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1410] Tcl / regsub - changing a string and using interesting bits - (2007-10-27)
  [1412] Sparse and Greedy matching - Tcl 8.4 - (2007-10-27)
  [4205] Regular Expression Substitution - Tcl - (2013-11-12)

Q803 - Object Orientation and General technical topics - Regular Expressions - Extra Elements
  [1372] A taster PHP expression ... - (2007-09-30)
  [1601] Replacing the last comma with an and - (2008-04-04)
  [1735] Finding words and work boundaries (MySQL, Perl, PHP) - (2008-08-03)
  [1860] Seven new intermediate Perl examples - (2008-10-30)
  [2909] Be gentle rather than macho ... regular expression techniques - (2010-08-08)
  [3089] Python regular expressions - repeating, splitting, lookahead and lookbehind - (2010-12-17)
  [3100] Looking ahead and behind in Regular Expressions - double matching - (2010-12-23)
  [3516] Regular Expression modifiers in PHP - summary table - (2011-11-12)
  [3650] Possessive Regular Expression Matching - Perl, Objective C and some other languages - (2012-03-12)

H107 - String Handling in PHP
  [31] Here documents - (2004-08-28)
  [54] PHP and natural sorting - (2004-09-19)
  [337] the array returned by preg_match_all - (2005-06-06)
  [422] PHP Magic Quotes - (2005-08-22)
  [463] Splitting the difference - (2005-10-13)
  [493] Running a Perl script within a PHP page - (2005-11-12)
  [558] Converting between acres and hectares - (2006-01-08)
  [560] The fencepost problem - (2006-01-10)
  [574] PHP - dividing a string up into pieces - (2006-01-23)
  [589] Robust PHP user inputs - (2006-02-03)
  [608] Don't expose your regular expressions - (2006-02-15)
  [642] How similar are two words - (2006-03-11)
  [716] Evaluating arithmetic expressions in configuration files - (2006-05-10)
  [728] Looking ahead and behind in a Regular Expression - (2006-05-22)
  [1008] Date conversion - PHP - (2006-12-26)
  [1058] PHP Regular expression to extrtact link and text - (2007-01-31)
  [1533] Short and sweet and sticky - PHP form input - (2008-02-06)
  [1603] Do not SHOUT and do not whisper - (2008-04-06)
  [1799] Regular Expressions in PHP - (2008-09-16)
  [2046] Finding variations on a surname - (2009-02-17)
  [2165] Making Regular Expressions easy to read and maintain - (2009-05-10)
  [2238] Handling nasty characters - Perl, PHP, Python, Tcl, Lua - (2009-06-14)
  [2629] Curly braces within double quoted strings in PHP - (2010-02-09)
  [3020] Handling (expanding) tabs in PHP - (2010-10-29)
  [3424] Divide 10000 by 17. Do you get 588.235294117647, 588.24 or 588? - Ruby and PHP - (2011-09-08)
  [3515] PHP - moving from ereg to preg for regular expressions - (2011-11-11)
  [3534] Learning to program in PHP - Regular Expression and Associative Array examples - (2011-12-01)
  [3788] Getting more than a yes / no answer from a regular expression pattern match - (2012-06-30)
  [3789] More than just matching with a regular expression in PHP - (2012-06-30)
  [3790] Solution looking for a problem? Lookahead and Lookbehind - (2012-06-30)
  [4071] Setting up strings in PHP - (2013-04-27)
  [4072] Splitting the difference with PHP - (2013-04-27)

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