Using Regular Expressions Metacharacters with .net – Greedy and Lazy quantifier

When a regular expression has a quantifier that can accept a range of repetitions (like ".*"), the normal behavior is to match as many characters as possible. Consider the following regular expression:

a.*b The longest string starting with a and ending with b

If this is used to search the string "aabab", it will match the entire string "aabab". This is called "greedy" matching. Sometimes, we prefer "lazy" matching in which a match using the minimum number of repetitions is found. All the quantifiers in Table 2 can be turned into "lazy" quantifiers by adding a question mark "?". Thus "*?" means "match any number of repetitions, but use the smallest number of repetitions that still leads to a successful match". Now let’s try the lazy version of example:

a.*?b The shortest string starting with a and ending with b

If we apply this to the same string "aabab" it will first match "aab" and then "ab".

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Using Regular Expressions Metacharacters with .net – Captures

(exp)

(exp) – Match exp and capture it in an automatically numbered group. Common, unadorned parentheses generally perform two functions, grouping and capturing. Common parentheses are almost always of the form (), but a few flavors use \(\).

(?<name>exp)

(?<name>exp) – Match exp and capture it in a group named name. .NET languages support captures to named locations.

(?:exp)

(?:exp) – Match exp, but do not capture it, but, as the name implies, group regex components for alternation and the application of quantifiers. They are not counted as part of $1, $2, etc. After a match of (1|one)(?:and|or)(2|two), for example, $1 contains ’1′ or ‘one’, while $2 contains ’2′ or ‘two’. Grouping-only parentheses are also called non-capturing parentheses.

Using Regular Expressions Metacharacters with .net – Comments and Mode Modifiers

With many flavors, the regex modes and match modes described earlier can be modified within the regex by the following constructs.

Mode modifier: (?modifier), such as (?i) or (?-i)

Many flavors now allow some of the regex and match modes to be set within the regular expression itself. A common example is the special notation (?i), which turns on case-insensitive matching, and (?-i), which turns it off. For example, <B>(?i)very(?-i)</B> has the very part match with case insensitivity, while still keeping the tag names case-sensitive. This matches ‘<B>VERY</B>’ and ‘<B>Very</B>’, for example, but not ‘<b>Very</b>’.

This example works with most systems that support (?i), including Perl, PHP, java.util.regex, Ruby,[] and the .NET languages. It doesn’t work with Python or Tcl, neither of which support (?-i).

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Using Regular Expressions Metacharacters with .net – Lookahead and Lookbehind Zero-Width Assertions

Perl 5 introduced two very powerful constructs: "lookahead" and "lookbehind". Collectively, these are called "lookaround". They are zero-width just like other "zero-width assertions" don’t match actual text, but rather positions in the text. The difference is that lookarounds will actually match characters, but then give up the match and only return the result: match or no match. That is why they are called "assertions". They do not consume characters in the string, but only assert whether a match is possible or not. Lookarounds allow you to create regular expressions that are impossible to create without them, or that would get very longwinded without them.

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Using Regular Expressions Metacharacters with .net – Anchors and Other "Zero-Width Assertions"

Anchors and other "zero-width assertions" don’t match actual text, but rather positions in the text.

^, \A

Caret (^) matches at the beginning of the text being searched, and, if in an enhanced line-anchor match mode, after any newline. In some systems, an enhanced-mode ^ can match after Unicode line terminators, as well. While \A always matches only at the start of the text being searched, regardless of any match mode.

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