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Regular Expressions

Posted by enquirer (enquirer), 20 February 2004
Does Tcl support regular expressions?

Posted by admin (Graham Ellis), 20 February 2004
Yes - if you're familiar with regular expressions in other languages, all versions of Tcl support the basics.  Versions 8.3 and later support more advanced regular expressions too, based on the POSIX rather than the Perl extensions.

Would you like a further explanation?

A regular expression is a pattern against which you can match a character string - you don't say "is the string exactly xxxx" but rather "does this string look like yyyyyy" where yyyyyy is a description written in a special format.   At its most straightforward, a regular expression match command return either a true ("yes, I matched") or false ("no, I didn't match") result.  It can be extended to return pieces of the incoming string that matched the particular pattern, and also to substitute matching parts of the incoming string with a replacement string.

Here's an example of a regular expression match in Tcl - this particular piece of code is a quick test to see if a line of test typed in by the user looks like an email address or not:

Code:
puts "please enter your email address: "
gets stdin line

if {[regexp {[^ ]+@([^ ]+)} $line all parts]} {
       puts "Yep, that's one $all from $parts"
} else {
       puts "No - don't think so!"
}


Regular expressions comprise about 6 different element types

Literals - characters that match exactly such as @ in the example
Character Classes  - where any one character from a number of possibilities is matched.  In the example [^ ] matches any character except a space
Counts - which specify that the previous element occurs a number of times.  We've used + to say "1 or more" in the example
Grouping - round brackets around part of a regular expression to indicate that this part is an 'interesting bit' to be saved out to a variable on successful match.  Groupings also effect counts and alternation (later!)
Assertions - You can start a regular expression with a ^ to indicate it must match at the start of the string, and/or end it with $ to indicate that the match must be at the end.  We have not used this in our example
Alternation - a | character can be used to indicate "or" - for example "rabbit|banana" will match rabbit or banana.



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