Implementing Regular Expressions in Programming Languages

Introduction

Regular expressions (regex), often referred to as regex, are sequences of characters used to match patterns within text. These patterns can be used to search, replace, and validate strings. Regular expressions have been part of programming since the 1960s, evolving and becoming more powerful with time. In this article, we will delve into the implementation of regular expressions in modern programming languages.

What is a Regular Expression?

A regular expression is a pattern matching tool that allows users to search for specific patterns within text. They are commonly used in search engines, programming languages, and input validation. For example, the regular expression customi[sz]able matches both "customizable" and "customisable" by using character classes

Implementing Regular Expressions in Programming Languages

The process of implementing regular expressions in a programming language consists of three main steps: determining the functionality, representing it, and implementing it either through built-in features or external libraries.

Step 1: Determine the Functionality

When implementing regular expressions, the first question to ask is what functionality you want to include. For instance, you might want to emulate the original regular expression functionality from ATT Unix in tools like awk, grep, and sed. Alternatively, you could extend the functionality as in Perl regular expressions.

Step 2: Represent the Functionality

The second step is to decide how to represent the regular expression functionality. This could be as a series of function calls, as is the case with languages like C, Java, and Python. Alternatively, you could incorporate the regex expressions directly into the language syntax, as Perl and awk do.

Step 3: Implement the Functionality

The final step is to determine whether to use an existing implementation or write your own. Most programming languages have built-in regex capabilities or libraries. However, if you need to write your own, ensure you have a very good reason.

Historical Context

The early origins of regular expressions trace back to the late 1950s and early 1960s. They were popularized by Ken Thompson in 1968 when he implemented them for IBM mainframes using Fortran. Thompson's work was later reimplemented in BCPL and included in the QED text editor.

Ken Thompson, along with Dennis Ritchie, was an ACM Turing Award winner for their work on UNIX. They later rewrote the QED editor into the UNIX ed editor. It was suggested by Mike Lesk to pull the 'global regular expression print' command from ed to create the UNIX grep utility.

For a detailed look at Ken Thompson's methods, you can refer to his paper on the Regular Expression Search Algorithm. Additionally, there are comprehensive resources available on how regular expression matching is implemented, such as this article and these lecture notes.

Conclusion

Regular expressions are a versatile tool with a rich history and continue to play a vital role in programming. By understanding their implementation, developers can better leverage these powerful tools to solve complex text-based problems.