{"id":1020,"date":"2013-12-06T00:09:34","date_gmt":"2013-12-06T05:09:34","guid":{"rendered":"http:\/\/osric.com\/chris\/accidental-developer\/?p=1020"},"modified":"2013-12-06T00:09:34","modified_gmt":"2013-12-06T05:09:34","slug":"using-group-expressions-in-regular-expression-pattern-matching","status":"publish","type":"post","link":"https:\/\/osric.com\/chris\/accidental-developer\/2013\/12\/using-group-expressions-in-regular-expression-pattern-matching\/","title":{"rendered":"Using group expressions in regular expression pattern matching"},"content":{"rendered":"

I’ve used group expressions in regexes many times, but only for replacement. Yesterday I learned that they can also be used for matching.<\/p>\n

For example, let’s say you have the text:<\/p>\n

Banananananas don’t grow in Mississississippi because banananas are afraid of getting turned into Missississippi’s famous bananana pudding.<\/p><\/blockquote>\n

The following regular expression will find instances of iss<\/em> or an<\/em> that are repeated more than twice.<\/p>\n

(iss|an)\\1\\1+<\/code><\/p>\n

You can use \\1\\1<\/code> as the replacement (or $1$1<\/code> in Dreamweaver, which uses backslashes to identify groups in match expressions, but dollar signs to represent groups in replace expressions) to turn the misspelled words into Mississippi<\/em> and banana(s)<\/em>.<\/p>\n

Another example might be applying consistent formatting to phone numbers or dates.<\/p>\n

Phone numbers<\/strong>
\nLet’s say you usually use 555-555-1212 as the format for phone numbers and sometimes you use 555.555.1212, but the new trend is to use spaces instead of dashes or dots as separators:<\/p>\n

Find:<\/strong> ([\\d]{3})([-\\.])([\\d]{3})\\2([\\d]{4})
\nReplace:<\/strong> \\1 \\3 \\4<\/p>\n

Dates<\/strong>
\nLet’s say you usually use 12\/5\/2013 as the format for dates, dabbled with 12.5.2013, but now you’ve decided that dashes are clearer:<\/p>\n

Find:<\/strong> ([\\d]{1,2})([\\.\/])([\\d]{1,2})\\2([\\d]{4})
\nReplace:<\/strong> \\1-\\3-\\4<\/p>\n

In both cases you could just repeat the bracketed character class, but then you could end up matching strings you didn’t intend to:<\/p>\n