Regular Expressions

This article is an introduction to an extremely powerful tool available to any programmer, Regular Expressions.

Testing

• The easiest way to test regular expressions (PCRE style) is probably to use Perl directly like this.

echo "input" | perl -n -e '/regexpHere/ and print'

• You can also use grep but it does not support PCRE syntax natively (GNU grep has the -P switch which does).

Operators

• ^ represents the start of the string to search, or a newline in multiline mode. It is optional. Note that if you use syntax such as '^.*', you can just remove it altogether: it's better just not to use ^ in this case.
• $ delimits the end of the string or just before the newline in multiline mode. It is optional.
• *? is the non-greedy operator, it will match as little text as possible. +?, ??, or {m,n}? are also available.
• ? allows to match optionally only one expression. It should be put after the expression, eg ab? will match either a or ab.

Flags

• Normally the "." special character matches any character except newlines. You can activate matching of newline with the DOTALL flag. In a regexp this is activated via (?s).

Quoting part of a regular expression

• This can be done via the \Q and \E constructs (you nest the literal inside those).

Regular expressions in Python

Official documentation available here.

Basic Operations

You can perform two basic operations: search and match. In Perl, search is always used.

• match(): Determine if the RE matches at the beginning of the string.
• search(): Scan through a string, looking for any location where this RE matches.

Groups

In a regular expression, we often want to extract a particular piece of information from a string. We need to enclose the relevant "sub expression" in parenthesis. In Python, we can then refer to this group by its number, or, if we add ?P<name> to the group, by its name. To create a group which will not be available later for retrieval, write (?:expression).

• Example:
  • regular_expression = re.compile(r"&price=(?P<Price>.*)&quantity=(?P<Quantity>.*)")
  • math_object = regular_expression.search(query_string)
  • We could access the price value by math_object.group(1) or math_object.group('Price'). For the quantity, it would be group(2) or group('Quantity').

Regular expressions in Java

• Documentation for the Pattern and Matcher classes.

• If you use a string to create a regular expression, you need to double escape special characters, eg \\*. This is because the first backslash is needed to escape the backslash itself in Java.
• The matches() method of the Matcher class is similar to match() in Python; find() is similar to search().
• Pattern.quote() is a static method that will produce a literal regular expression out of a string. This is useful for matching literally strings including characters such as "*" for example.

Regular expressions in Groovy

• Documentation.

• Groovy has the following shortcuts:
  • ==~ for matches().
  • =~ for creating a matcher. The matcher is coerced to a Boolean via its find() method, thus you can write stuff like

if ("hello" =~ /hel/)

Be careful to include parenthesis in the following case:

if (! ("hello" =~ /hal/))

• A pattern can be directly created via ~/foo/.