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Regular Expressaion (RegExps) are used for matching pieces of text. In perl a RegExp consists of delimiters, a pattern and some optional settings. There are three types of RegExp in perl: match, replace and transliterate; we'll look at each in turn.
The simple matching RegExp checks to see whether a pattern can be found in a string. For the moment we'll use '/' as our delimiter. Here's an example:
$my_variable =~ m/hello/i;
Lets' break that down and see what we have...
$my_variables =~
Perform a regexp on $my_variable.
m/
This is a matching regexp (if '/' is used as the delimiter, the 'm' can be ommited)
hello
The pattern to match - just a simple string in this case.
/i;
End of the pattern, and the 'i' option, which means the RegExp will be case insensitive.
Because $_ is the 'default variable' in perl, you can perform a regexp on it as follows:
/hello/i;
so if you see this sort of thing on it's own, then it's matching against $_.
The format of the pattern doesn't just have to contain alpha numerics. RegExps have 'meta-characters' which are not taken as literals. These are the following characters: "+ ? . * ^ $ ( ) [ ] { } | \". To use one of these characters literally, escape it with a slash. If you want to match "file.txt", use:
$var =~ h/file\.txt/;
Note: The delimiter you use also becomes a metachar, so always escape it if you want it to be literal! We'll go through each metachar, looking at what it does:
+ matches the preceeding group one or more times
* matches the preceeding group zero or more times
? matches the preceeding group zero or one times
{a} matches the preceeding group a times
{a,} matches the preceeding group a or more times
{a,b} matches the preceeding group between a and b times
a group is defined as either a single character, or a group of chars surrounded with brackets "()". A demonstration:
/bob+/ will match bob, bobb, bobbb /(bob)+/ will match bob, bobbob, bobbob
The dot "." meta character matches any character, EXCEPT newlines. So:
/.*/ matches anything, excluding newlines /a./ matches aa, ab, ac, a*, a:
To make the dot match newline characters aswell, use single line 's' mode:
/.*/s matches anything, including newlines
The caret "^" and dollar "$" signs mark the beginning and end of a line respectively. So this...
/^the only thing in the string$/
...checks to see if the expression matches the entire line.
The square brackets are used to denote a "character class", which is a group of characters to choose from:
/x[abc]x/ matches xax, xbx, xcx
A class can be used with other metachars too:
/[abc]+/ matches b, acb, cabcbabcbab
The special case for classes is when they start with a caret "^". This negates the class, meaning it matches anything EXCEPT the characters within:
/[^aeiou]/ matches any non vowel character.
The | metachar is used to give alternatives:
/a|b|c/ matches a, b, c
It can also be used with groups, so:
/(bob)|(jack)|[abc]/ matches bob, jack, a, b, c