3.1 Fundamental Structure

The fundamental building blocks are the regular expressions that match a single character. Most characters, including all letters and digits, are regular expressions that match themselves. Any meta-character with special meaning may be quoted by preceding it with a backslash.

A regular expression may be followed by one of several repetition operators:

‘.’

The period ‘.’ matches any single character.

‘?’

The preceding item is optional and will be matched at most once.

‘*’

The preceding item will be matched zero or more times.

‘+’

The preceding item will be matched one or more times.

‘{n}’

The preceding item is matched exactly n times.

‘{n,}’

The preceding item is matched n or more times.

‘{,m}’

The preceding item is matched at most m times. This is a GNU extension.

‘{n,m}’

The preceding item is matched at least n times, but not more than m times.

The empty regular expression matches the empty string. Two regular expressions may be concatenated; the resulting regular expression matches any string formed by concatenating two substrings that respectively match the concatenated expressions.

Two regular expressions may be joined by the infix operator ‘|’; the resulting regular expression matches any string matching either alternate expression.

Repetition takes precedence over concatenation, which in turn takes precedence over alternation. A whole expression may be enclosed in parentheses to override these precedence rules and form a subexpression.

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