文字模式

In a regex, "normal" alphanumeric characters simply match themselves: write ciao as the pattern and the engine will look for the substring ciao inside the text. Nothing more magical: a left-to-right scan position by position.

Pattern: ciao
Sample:  Buongiorno, ciao mondo! Ti dico anche ciao.
                     ^^^^                        ^^^^

A match always carries two key pieces of information:

• the matched text (here ciao);
• the index (0-based offset) where it starts in the sample (here 12 and 38).

By default regex are case-sensitive: ciao does not match Ciao or CIAO. To ignore case add the i flag (case-insensitive).

How the engine works

The regex engine inspects the text one character at a time. When searching for the literal pattern ciao, it first looks for the letter c. Once found, it verifies if the next character is i, then a, and finally o. If any of these matches fail, the engine backtracks to the next starting position and starts looking for c again.

Literal patterns and the g flag

Without the g (global) flag the engine stops at the first match it finds and halts. With g it continues and collects every subsequent match until the end of the string. In this course we will almost always turn g on: we want to see every match present in the text.

Try it

Review exercise

Additional challenge