Right way to escape backslash [ \ ] in PHP regex?

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// PHP 5.4.1

// Either three or four \ can be used to match a '\'.
echo preg_match( '/\\\/', '\\' );        // 1
echo preg_match( '/\\\\/', '\\' );       // 1

// Match two backslashes `\\`.
echo preg_match( '/\\\\\\/', '\\\\' );   // Warning: No ending delimiter "https://stackoverflow.com/" found
echo preg_match( '/\\\\\\\/', '\\\\' );  // 1
echo preg_match( '/\\\\\\\\/', '\\\\' ); // 1

// Match one backslash using a character class.
echo preg_match( '/[\\]/', '\\' );       // 0
echo preg_match( '/[\\\]/', '\\' );      // 1  
echo preg_match( '/[\\\\]/', '\\' );     // 1

When using three backslashes to match a '\' the pattern below is interpreted as match a '\' followed by an 's'.

echo preg_match( '/\\\\s/', '\\ ' );    // 0  
echo preg_match( '/\\\\s/', '\\s' );    // 1

When using four backslashes to match a '\' the pattern below is interpreted as match a '\' followed by a space character.

echo preg_match( '/\\\\\s/', '\\ ' );   // 1
echo preg_match( '/\\\\\s/', '\\s' );   // 0

The same applies if inside a character class.

echo preg_match( '/[\\\\s]/', ' ' );   // 0 
echo preg_match( '/[\\\\\s]/', ' ' );  // 1

None of the above results are affected by enclosing the strings in double instead of single quotes.

Conclusions:
Whether inside or outside a bracketed character class, a literal backslash can be matched using just three backslashes '\\\' unless the next character in the pattern is also backslashed, in which case the literal backslash must be matched using four backslashes.

Recommendation:
Always use four backslashes '\\\\' in a regex pattern when seeking to match a backslash.

Escape sequences.

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