Digital Tools for Research

Symbol Ranges

You can combine ranges:

Tip: regular expressions operate Unicode symbol ranges, and you can create custom ones using Unicode blocks as reference.

Groups & Backreferencing

A part of a pattern can be enclosed in parentheses. This is called a capturing group. You can later refer to this group by its number, for example, when you need to swap chunks of text.

Groups are numbered by the opening parenthesis.

Here is an example of swapping AB and BA using capturing groups:

Quantifiers

"Greedy" and "lazy" quantifiers

Quantifiers by default behave greedily: this means that they try to "consume" as many characters as possible and, out of all possible options, return the longest string. To make a quantifier "lazy", i.e. matching the shortest possible string, you need to add a ? after that quantifier.

Be cautious!

Regex can be very helpful, but you can also easily ruin your data with them — especially if you are bulk-processing many files. Always double-check your regular expression on test data before making irreversible changes!

The most dangerous pattern is .*, which reads as any character any times from 0 to infinity, as many times as possible (greedy). By itself, it will just match a whole string! The "any character any times" bit doesn't mean that the same character has to be repeated. The quantifier applies to the regex element ("any character"), not to a particular match!

Special Characters

Anchors

Escaping Syntax Elements

As you've already noticed, like any language, regular expressions are written using a special alphabet—dots, asterisks, parentheses, etc. But what if you need to find special characters like + or * in the text? It's simple: you need to escape them by placing a backslash before them.

NB! Within square brackets (a range), syntax elements lose their power, and you don't have to escape them.

• .*? — matches any character any times from 0 to infinity, as few times as possible (lazy)
• \.\*\? — matches a sequence of 3 characters .*? literally
• [.*?]— matches a single character that can be either a dot, or an asterisk, or a question mark

• David Child's Regex Cheatsheet
• DataCamp Regex Cheatsheet

When using these websites, you'll see both the matches and a detailed explanation of your regular expression.

• https://regex101.com/
• http://regexr.com/
• http://myregexp.com/
• https://www.regexpal.com/

• https://regexcrossword.com/
• https://regexone.com/

This website provides a library of regular expressions, which you can search by keywords, e.g. 'time': https://regexlib.com/

• Regex in Google Docs
• Regex in Python
• Regex in R
• Regex in Java
• Regex in JavaScript