If you want to say does this string "look like" another, without being able to give an explicit string for the 'another', then you're probably looking for a regular expression.


Regular expressions come in various flavours, and the flavour used in Ruby is "Perl Style" (i.e. it's similar to Perl, Python and the preg functions in PHP, and differs from Tcl and the ereg functions in PHP).

In summary, a regular expression comprises elements such as:

Literals - exact matches
 - letters, digits and some special characters match exactly
 - special characters can be matched when preceded by a \

Character groups - any one character from a selection
 - [abcyh] any character from the list
 - [^abcyh] any character not in the list
 - [A-Z0-4] any capital letter of digit 0 through 4
 - \s \d \w any space, digit or word character
 - \S \D \W any non-space, non-digit or non-word character
 - . any character at all (may not match \n)

Counts - apply to previous literal, character or group
 - {2,6} 2 to 6 occurrences of previous item
 - {4,} 4 or more occurrences of previous item
 - {5} exactly 5 occurrences of previous item
 - + 1 or more of previous item
 - * 0 or more of previous item
 - ? 0 or 1 of previous item

Anchors (a.k.a. Zero width assertions)
 - ^ match here at start of string or line
 - $ match here at end of string or line
 - \b match here at word boundary

Miscellany
 - ( .... ) grouping for capture and counting
 - | alternation; "either / or"
 - \1 \2 references back to previous groups.

There are more options - but those are the common ones.


The =~ operator is the 'match' operator, so I can ask if something looks like a regular expression. The index method on a string will recognise a regular expression and use it to separate. The variable $& contains the matched string, and $1, $2 etc contain matched capture groups.

Example:


Here is the result of running that program:


Ruby also allows you to produce compiled regular expression objects, and matchdata objects, and use those for more efficient and more sophisticated matching. And methods such as split, too, can use regular expressions.


Results:


Although we've been using / delimiters for regular expressions, you can if you prefer user %r! through ! (or replace the ! with any other special character) in much the same was as %Q and %q


See also Programming in Ruby - Course

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Ruby - Strings and Regular Expressions
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At Well House Consultants, we provide training courses on subjects such as Ruby, Lua, Perl, Python, Linux, C, C++, Tcl/Tk, Tomcat, PHP and MySQL. We're asked (and answer) many questions, and answers to those which are of general interest are published in this area of our site.