MYSQL error : #3699 - Timeout exceeded in regular expression match

SET GLOBAL regexp_time_limit=1024;

(see here for the documentation)

You should make the regex into a trie.
This is less work for the engine, it only has to do at most 16 steps to fail
instead of the current 110 steps it's doing now.

Try

A(?:achen|ugsburg)|B(?:a(?:den-Württemberg|varia|yern)|er(?:gisch[ ]Gladbach|lin)|ielefeld|o(?:chum|nn|ttrop)|r(?:a(?:ndenbur|unschwei)g|eme(?:rhave)?n))|C(?:hemnitz|ologne)|D(?:armstadt|eutsch|ortmund|resden|uisburg|üsseldorf)|E(?:r(?:furt|langen)|ssen)|F(?:r(?:ankfurt|eiburg)|ürth)|G(?:e(?:lsenkirche|rma)n|öttingen)|H(?:a(?:gen|lle|m(?:burg|m)|n(?:n)?over)|e(?:i(?:delberg|lbronn)|rne|ssen?)|ildesheim)|Ingolstadt|Jena|K(?:a(?:rlsruhe|ssel)|iel|oblenz|refeld|öln)|L(?:e(?:ipzig|verkusen)|ower[ ]Saxony|udwigshafen|übeck)|M(?:a(?:gdeburg|inz|nnheim)|ecklenburg-Vorpommern|oers|unich|önchengladbach|ü(?:lheim|n(?:chen|ster)))|N(?:euss|iedersachsen|or(?:drhein-Westfalen|th[ ]Rhine-)|uremberg|ürnberg)|O(?:berhausen|ffenbach|ldenburg|snabrück)|P(?:aderborn|forzheim|otsdam)|R(?:e(?:cklinghausen|gensburg|mscheid|utlingen)|h(?:einland-Pfalz|ineland-Palatinate)|ostock)|S(?:a(?:a(?:le|r(?:brücken|land))|chsen(?:-Anhalt)?|lzgitter|xony(?:-Anhalt)?)|chleswig-Holstein|iegen|olingen|tuttgart)|T(?:h(?:uringia|üringen)|rier)|Ulm|W(?:estphalia|iesbaden|olfsburg|uppertal|ürzburg)

Change the (?: to ( if your engine doesn't support non-capture syntax.
Also, you can change [ ] to just space if you want to on the compressed
regex.

https://regex101.com/r/iEzw33/1

Expanded

    A
    (?: achen | ugsburg )
 |  B
    (?:
         a
         (?: den-Württemberg | varia | yern )
      |  er
         (?: gisch [ ] Gladbach | lin )
      |  ielefeld
      |  o
         (?: chum | nn | ttrop )
      |  r
         (?:
              a
              (?: ndenbur | unschwei )
              g
           |  eme
              (?: rhave )?
              n
         )
    )
 |  C
    (?: hemnitz | ologne )
 |  D
    (?:
         armstadt
      |  eutsch
      |  ortmund
      |  resden
      |  uisburg
      |  üsseldorf
    )
 |  E
    (?:
         r
         (?: furt | langen )
      |  ssen
    )
 |  F
    (?:
         r
         (?: ankfurt | eiburg )
      |  ürth
    )
 |  G
    (?:
         e
         (?: lsenkirche | rma )
         n
      |  öttingen
    )
 |  H
    (?:
         a
         (?:
              gen
           |  lle
           |  m
              (?: burg | m )
           |  n
              (?: n )?
              over
         )
      |  e
         (?:
              i
              (?: delberg | lbronn )
           |  rne
           |  ssen?
         )
      |  ildesheim
    )
 |  Ingolstadt
 |  Jena
 |  K
    (?:
         a
         (?: rlsruhe | ssel )
      |  iel
      |  oblenz
      |  refeld
      |  öln
    )
 |  L
    (?:
         e
         (?: ipzig | verkusen )
      |  ower [ ] Saxony
      |  udwigshafen
      |  übeck
    )
 |  M
    (?:
         a
         (?: gdeburg | inz | nnheim )
      |  ecklenburg-Vorpommern
      |  oers
      |  unich
      |  önchengladbach
      |  ü
         (?:
              lheim
           |  n
              (?: chen | ster )
         )
    )
 |  N
    (?:
         euss
      |  iedersachsen
      |  or
         (?: drhein-Westfalen | th [ ] Rhine- )
      |  uremberg
      |  ürnberg
    )
 |  O
    (?:
         berhausen
      |  ffenbach
      |  ldenburg
      |  snabrück 
    )
 |  P
    (?: aderborn | forzheim | otsdam )
 |  R
    (?:
         e
         (?:
              cklinghausen
           |  gensburg
           |  mscheid
           |  utlingen 
         )
      |  h
         (?: einland-Pfalz | ineland-Palatinate )
      |  ostock
    )
 |  S
    (?:
         a
         (?:
              a
              (?:
                   le
                |  r
                   (?: brücken | land )
              )
           |  chsen
              (?: -Anhalt )?
           |  lzgitter
           |  xony
              (?: -Anhalt )?
         )
      |  chleswig-Holstein
      |  iegen
      |  olingen
      |  tuttgart
    )
 |  T
    (?:
         h
         (?: uringia | üringen )
      |  rier
    )
 |  Ulm
 |  W
    (?:
         estphalia
      |  iesbaden
      |  olfsburg
      |  uppertal
      |  ürzburg
    )

Lets check the speed:

Regex1:   A(?:achen|ugsburg)|B(?:a(?:den-Württemberg|varia|yern)|er(?:gisch[ ]Gladbach|lin)|ielefeld|o(?:chum|nn|ttrop)|r(?:a(?:ndenbur|unschwei)g|eme(?:rhave)?n))|C(?:hemnitz|ologne)|D(?:armstadt|eutsch|ortmund|resden|uisburg|üsseldorf)|E(?:r(?:furt|langen)|ssen)|F(?:r(?:ankfurt|eiburg)|ürth)|G(?:e(?:lsenkirche|rma)n|öttingen)|H(?:a(?:gen|lle|m(?:burg|m)|n(?:n)?over)|e(?:i(?:delberg|lbronn)|rne|ssen?)|ildesheim)|Ingolstadt|Jena|K(?:a(?:rlsruhe|ssel)|iel|oblenz|refeld|öln)|L(?:e(?:ipzig|verkusen)|ower[ ]Saxony|udwigshafen|übeck)|M(?:a(?:gdeburg|inz|nnheim)|ecklenburg-Vorpommern|oers|unich|önchengladbach|ü(?:lheim|n(?:chen|ster)))|N(?:euss|iedersachsen|or(?:drhein-Westfalen|th[ ]Rhine-)|uremberg|ürnberg)|O(?:berhausen|ffenbach|ldenburg|snabrück)|P(?:aderborn|forzheim|otsdam)|R(?:e(?:cklinghausen|gensburg|mscheid|utlingen)|h(?:einland-Pfalz|ineland-Palatinate)|ostock)|S(?:a(?:a(?:le|r(?:brücken|land))|chsen(?:-Anhalt)?|lzgitter|xony(?:-Anhalt)?)|chleswig-Holstein|iegen|olingen|tuttgart)|T(?:h(?:uringia|üringen)|rier)|Ulm|W(?:estphalia|iesbaden|olfsburg|uppertal|ürzburg)
Completed iterations:   50  /  50     ( x 1000 )
Matches found per iteration:   110
Elapsed Time:    5.74 s,   5736.58 ms,   5736578 µs
Matches per sec:   958,759

Wow, a million words matched per second !

"Timeout" is presumably due to a slow query against a large table. So, let's address the speed of the query.

• REPLACE is DELETE + INSERT.
• REPLACE is slow due to theDELETE. And it will loseterm_order` if you have already set it.
• Consider using INSERT IGNORE instead.
• The WHERE clause, due to the OR and the REGEXP, will need to scan the entire table. A FULLTEXT index could avoid that; see below.
• Refactoring the REGEXP won't help much; changing to LIKE is likely to be slower; etc.
• If the timeout is truely inside the REGEXP engine, then you may need to run the query a few cities at a time.

FULLTEXT would greatly improve the speed. But, if you don't want to go that direction, the next best is changing REPLACE to INSERT IGNORE.

The FULLTEXT approach is:

ALTER TABLE wp_posts ADD FULLTEXT(post_title, post_content);  -- one-time task

INSERT IGNORE INTO ... (...)
    SELECT 272, ID
        FROM wp_posts
        WHERE MATCH(post_title, post_content)
              AGAINST('German Deutsch Berlin Hamburg Munich München ...' IN BOOLEAN MODE)

(There may be an issue with handling of accents, depending on the version of MySQL and which ENGINE is used.)