File: grep.info,  Node: Fundamental Structure,  Next: Character Classes and Bracket Expressions,  Up: Regular Expressions

3.1 Fundamental Structure
=========================

In regular expressions, the characters ‘.?*+{|()[\^$’ are “special
characters” and have uses described below.  All other characters are
“ordinary characters”, and each ordinary character is a regular
expression that matches itself.

   The period ‘.’ matches any single character.  It is unspecified
whether ‘.’ matches an encoding error.

   A regular expression may be followed by one of several repetition
operators; the operators beginning with ‘{’ are called “interval
expressions”.

‘?’
     The preceding item is optional and is matched at most once.

‘*’
     The preceding item is matched zero or more times.

‘+’
     The preceding item is 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 of the
two expressions, which are called “alternatives”.

   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.
An unmatched ‘)’ matches just itself.

   Not every character string is a valid regular expression.  *Note
Problematic Expressions::.