Asn1Browser v4.4

Regular expressions

Contents

• Introduction
• Syntax
  • Positional Operators
  • One-Character Operators
  • Character Class Operator
  • Subexpressions and Backreferences
  • Branching (Alternation) Operator
  • Repeating Operators
• Reference Material

Introduction

This is a short overview of regular expressions used to define file filters in profile files of Asn1Browser.

A regular expression consists of a character string where some characters are given special meaning with regard to pattern matching. Regular expressions have been in use from the early days of computing, and provide a powerful and efficient way to parse, interpret and search and replace text within an application.

Syntax

Within a regular expression, the following characters have special meaning.

Positional Operators

• ^ matches at the beginning of a line.
• $ matches at the end of a line.
• \A matches the start of the entire string.
• \Z matches the end of the entire string.

One-Character Operators

• . matches any single character.
• \d matches any decimal digit.
• \D matches any non-digit.
• \n matches a newline character.
• \r matches a return character.
• \s matches any whitespace character.
• \S matches any non-whitespace character.
• \t matches a horizontal tab character.
• \w matches any word (alphanumeric) character.
• \W matches any non-word (alphanumeric) character.
• \x matches the character x, if x is not one of the above listed escape sequences.

Character Class Operator

• [abc] matches any character in the set a, b or c.
• [^abc] matches any character not in the set a, b or c.
• [a-z] matches any character in the range a to z, inclusive.
• A leading or trailing dash will be interpreted literally.

Subexpressions and Backreferences

• (abc)matches whatever the expression abc would match, and saves it as a subexpression. Also used for grouping.
• (?:...) pure grouping operator, does not save contents.
• (?#...) embedded comment, ignored by engine.
• \n where 0 < n < 10, matches the same thing the nth subexpression matched.

Branching (Alternation) Operator

• a|b matches whatever the expression a would match, or whatever the expression b would match.
  

Repeating Operators

These symbols operate on the previous atomic expression :

• ? matches the preceding expression or the null string.
• * matches the null string or any number of repetitions of the preceding expression.
• + matches one or more repetitions of the preceding expression.
• {m} matches exactly m repetitions of the one-character expression.
• {m,n} matches between m and n repetitions of the preceding expression, inclusive.
• {m, } matches m or more repetitions of the preceding expression.

Reference Material

We have used the package gnu.regexp - Regular Expressions for Java - to implement this function. You can refer to the site gnu.regexp to get more information about the regular expressions.